GROFF_DIFF(7)              OpenBSD Reference Manual              GROFF_DIFF(7)



NAME
       groff_diff - differences between GNU troff and classical troff

DESCRIPTION
       This manual page describes the language differences between groff, the
       GNU roff text processing system, and the classical roff formatter of
       the freely available Unix 7 of the 1970s, documented in the Troff
       User's Manual by Ossanna and Kernighan.  This inludes the roff language
       as well as the intermediate output format (troff output).

       The section SEE ALSO gives pointers to both the classical roff and the
       modern groff documentation.

GROFF LANGUAGE
       In this section, all additional features of groff compared to the
       classical Unix 7 troff are described in detail.

   Long names
       The names of number registers, fonts, strings/macros/diversions,
       special characters (glyphs), and colors can be of any length.  In
       escape sequences, additionally to the classical `(xx' construction for
       a two-character glyph name, you can use `[xxx]' for a name of arbitrary
       length.

       \[xxx] Print the special character (glyph) called xxx.

       \[comp1 comp2 ...]
              Print composite glyph consisting of multiple components.
              Example: `\[A ho]' is capital letter A with ogonek which finally
              maps to glyph name `u0041_0328'.  See the groff info file for
              details how a glyph name for a composite glyph is constructed,
              and groff_char(7) for a list of glyph name components used in
              composite glyph names.

       \f[xxx]
              Set font xxx.  Additionally, \f[] is a new syntax form equal to
              \fP, i.e., to return to the previous font.

       \*[xxx arg1 arg2 ...]
              Interpolate string xxx, taking arg1, arg2, ..., as arguments.

       \n[xxx]
              Interpolate number register xxx.

   Fractional point sizes
       A scaled point is equal to 1/sizescale points, where sizescale is
       specified in the DESC file (1 by default).  There is a new scale
       indicator z that has the effect of multiplying by sizescale.  Requests
       and escape sequences in troff interpret arguments that represent a
       point size as being in units of scaled points, but they evaluate each
       such argument using a default scale indicator of z.  Arguments treated
       in this way are the argument to the ps request, the third argument to
       the cs request, the second and fourth arguments to the tkf request, the
       argument to the \H escape sequence, and those variants of the \s escape
       sequence that take a numeric expression as their argument.

       For example, suppose sizescale is 1000; then a scaled point is
       equivalent to a millipoint; the call .ps 10.25 is equivalent to
       .ps 10.25z and so sets the point size to 10250 scaled points, which is
       equal to 10.25 points.

       The number register \n[.s] returns the point size in points as decimal
       fraction.  There is also a new number register \n[.ps] that returns the
       point size in scaled points.

       It would make no sense to use the z scale indicator in a numeric
       expression whose default scale indicator was neither u nor z, and so
       troff disallows this.  Similarly it would make no sense to use a
       scaling indicator other than z or u in a numeric expression whose
       default scale indicator was z, and so troff disallows this as well.

       There is also new scale indicator s which multiplies by the number of
       units in a scaled point.  So, for example, \n[.ps]s is equal to 1m.  Be
       sure not to confuse the s and z scale indicators.

   Numeric expressions
       Spaces are permitted in a number expression within parentheses.

       M indicates a scale of 100ths of an em.  f indicates a scale of 65536
       units, providing fractions for color definitions with the defcolor
       request.  For example, 0.5f = 32768u.

       e1>?e2 The maximum of e1 and e2.

       e1<?e2 The minimum of e1 and e2.

       (c;e)  Evaluate e using c as the default scaling indicator.  If c is
              missing, ignore scaling indicators in the evaluation of e.

   New escape sequences
       \A'anything'
              This expands to 1 or 0, depending on whether anything is or is
              not acceptable as the name of a string, macro, diversion, number
              register, environment, font, or color.  It returns 0 if anything
              is empty.  This is useful if you want to look up user input in
              some sort of associative table.

       \B'anything'
              This expands to 1 or 0, depending on whether anything is or is
              not a valid numeric expression.  It returns 0 if anything is
              empty.

       \C'xxx'
              Typeset glyph named xxx.  Normally it is more convenient to use
              \[xxx].  But \C has the advantage that it is compatible with
              recent versions of UNIX and is available in compatibility mode.

       \E     This is equivalent to an escape character, but it is not
              interpreted in copy mode.  For example, strings to start and end
              superscripting could be defined like this

                     .ds { \v'-.3m'\s'\En[.s]*6u/10u'
                     .ds } \s0\v'.3m'

              The use of \E ensures that these definitions work even if \*{
              gets interpreted in copy mode (for example, by being used in a
              macro argument).

       \Ff
       \F(fm
       \F[fam]
              Change font family.  This is the same as the fam request.  \F[]
              switches back to the previous color (note that \FP won't work;
              it selects font family `P' instead).

       \mx
       \m(xx
       \m[xxx]
              Set drawing color.  \m[] switches back to the previous color.

       \Mx
       \M(xx
       \M[xxx]
              Set background color for filled objects drawn with the \D'...'
              commands.  \M[] switches back to the previous color.

       \N'n'  Typeset the glyph with index n in the current font.  n can be
              any integer.  Most devices only have glyphs with indices between
              0 and 255.  If the current font does not contain a glyph with
              that code, special fonts are not searched.  The \N escape
              sequence can be conveniently used in conjunction with the char
              request, for example

                     .char \[phone] \f(ZD\N'37'

              The index of each glyph is given in the fourth column in the
              font description file after the charset command.  It is possible
              to include unnamed glyphs in the font description file by using
              a name of ---; the \N escape sequence is the only way to use
              these.

       \On
       \O[n]  Suppress troff output.  The escapes \O2, \O3, \O4, and \O5 are
              intended for internal use by grohtml.

              \O0    Disable any ditroff glyphs from being emitted to the
                     device driver, provided that the escape occurs at the
                     outer level (see \O3 and \O4).

              \O1    Enable output of glyphs, provided that the escape occurs
                     at the outer level.

                     \O0 and \O1 also reset the registers \n[opminx],
                     \n[opminy], \n[opmaxx], and \n[opmaxy] to -1.  These four
                     registers mark the top left and bottom right hand corners
                     of a box which encompasses all written glyphs.

              \O2    Provided that the escape occurs at the outer level,
                     enable output of glyphs and also write out to stderr the
                     page number and four registers encompassing the glyphs
                     previously written since the last call to \O.

              \O3    Begin a nesting level.  At start-up, troff is at outer
                     level.  This is really an internal mechanism for grohtml
                     while producing images.  They are generated by running
                     the troff source through troff to the postscript device
                     and ghostscript to produce images in PNG format.  The \O3
                     escape starts a new page if the device is not html (to
                     reduce the possibility of images crossing a page
                     boundary).

              \O4    End a nesting level.

              \O5[Pfilename]
                     This escape is grohtml specific.  Provided that this
                     escape occurs at the outer nesting level, write filename
                     to stderr.  The position of the image, P, must be
                     specified and must be one of l, r, c, or i (left, right,
                     centered, inline).  filename is associated with the
                     production of the next inline image.

       \R'name +-n'
              This has the same effect as

                     .nr name +-n

       \s(nn
       \s+-(nn
              Set the point size to nn points; nn must be exactly two digits.

       \s[+-n]
       \s+-[n]
       \s'+-n'
       \s+-'n'
              Set the point size to n scaled points; n is a numeric expression
              with a default scale indicator of z.

       \Vx
       \V(xx
       \V[xxx]
              Interpolate the contents of the environment variable xxx, as
              returned by getenv(3).  \V is interpreted in copy mode.

       \Yx
       \Y(xx
       \Y[xxx]
              This is approximately equivalent to \X'\*[xxx]'.  However the
              contents of the string or macro xxx are not interpreted; also it
              is permitted for xxx to have been defined as a macro and thus
              contain newlines (it is not permitted for the argument to \X to
              contain newlines).  The inclusion of newlines requires an
              extension to the UNIX troff output format, and confuses drivers
              that do not know about this extension.

       \Z'anything'
              Print anything and then restore the horizontal and vertical
              position; anything may not contain tabs or leaders.

       \$0    The name by which the current macro was invoked.  The als
              request can make a macro have more than one name.

       \$*    In a macro or string, the concatenation of all the arguments
              separated by spaces.

       \$@    In a macro or string, the concatenation of all the arguments
              with each surrounded by double quotes, and separated by spaces.

       \$^    In a macro, the representation of all parameters as if they were
              an argument to the ds request.

       \$(nn
       \$[nnn]
              In a macro or string, this gives the nn-th or nnn-th argument.
              Macros and strings can have an unlimited number of arguments.

       \?anything\?
              When used in a diversion, this transparently embeds anything in
              the diversion.  anything is read in copy mode.  When the
              diversion is reread, anything is interpreted.  anything may not
              contain newlines; use \! if you want to embed newlines in a
              diversion.  The escape sequence \? is also recognized in copy
              mode and turned into a single internal code; it is this code
              that terminates anything.  Thus

                     .nr x 1
                     .nf
                     .di d
                     \?\\?\\\\?\\\\\\\\nx\\\\?\\?\?
                     .di
                     .nr x 2
                     .di e
                     .d
                     .di
                     .nr x 3
                     .di f
                     .e
                     .di
                     .nr x 4
                     .f

              prints 4.

       \/     This increases the width of the preceding glyph so that the
              spacing between that glyph and the following glyph is correct if
              the following glyph is a roman glyph.  It is a good idea to use
              this escape sequence whenever an italic glyph is immediately
              followed by a roman glyph without any intervening space.

       \,     This modifies the spacing of the following glyph so that the
              spacing between that glyph and the preceding glyph is correct if
              the preceding glyph is a roman glyph.  It is a good idea to use
              this escape sequence whenever a roman glyph is immediately
              followed by an italic glyph without any intervening space.

       \)     Like \& except that it behaves like a character declared with
              the cflags request to be transparent for the purposes of end-of-
              sentence recognition.

       \~     This produces an unbreakable space that stretches like a normal
              inter-word space when a line is adjusted.

       \:     This causes the insertion of a zero-width break point.  It is
              equal to \% within a word but without insertion of a soft hyphen
              glyph.

       \#     Everything up to and including the next newline is ignored.
              This is interpreted in copy mode.  It is like \" except that \"
              does not ignore the terminating newline.

   New requests
       .aln xx yy
              Create an alias xx for number register object named yy.  The new
              name and the old name are exactly equivalent.  If yy is
              undefined, a warning of type reg is generated, and the request
              is ignored.

       .als xx yy
              Create an alias xx for request, string, macro, or diversion
              object named yy.  The new name and the old name are exactly
              equivalent (it is similar to a hard rather than a soft link).
              If yy is undefined, a warning of type mac is generated, and the
              request is ignored.  The de, am, di, da, ds, and as requests
              only create a new object if the name of the macro, diversion or
              string is currently undefined or if it is defined to be a
              request; normally they modify the value of an existing object.

       .am1 xx yy
              Similar to .am, but compatibility mode is switched off during
              execution.  To be more precise, a `compatibility save' token is
              inserted at the beginning of the macro addition, and a
              `compatibility restore' token at the end.  As a consequence, the
              requests am, am1, de, and de1 can be intermixed freely since the
              compatibility save/restore tokens only affect the macro parts
              defined by .am1 and .ds1.

       .ami xx yy
              Append to macro indirectly.  See the dei request below for more
              information.

       .ami1 xx yy
              Same as the ami request but compatibility mode is switched off
              during execution.

       .as1 xx yy
              Similar to .as, but compatibility mode is switched off during
              expansion.  To be more precise, a `compatibility save' token is
              inserted at the beginning of the string, and a `compatibility
              restore' token at the end.  As a consequence, the requests as,
              as1, ds, and ds1 can be intermixed freely since the
              compatibility save/restore tokens only affect the (sub)strings
              defined by as1 and ds1.

       .asciify xx
              This request `unformats' the diversion xx in such a way that
              ASCII and space characters (and some escape sequences) that were
              formatted and diverted into xx are treated like ordinary input
              characters when xx is reread.  Useful for diversions in
              conjunction with the writem request.  It can be also used for
              gross hacks; for example, this

                     .tr @.
                     .di x
                     @nr n 1
                     .br
                     .di
                     .tr @@
                     .asciify x
                     .x

              sets register n to 1.  Note that glyph information (font, font
              size, etc.) is not preserved; use .unformat instead.

       .backtrace
              Print a backtrace of the input stack on stderr.

       .blm xx
              Set the blank line macro to xx.  If there is a blank line macro,
              it is invoked when a blank line is encountered instead of the
              usual troff behaviour.

       .box xx
       .boxa xx
              These requests are similar to the di and da requests with the
              exception that a partially filled line does not become part of
              the diversion (i.e., the diversion always starts with a new
              line) but is restored after ending the diversion, discarding the
              partially filled line which possibly comes from the diversion.

       .break Break out of a while loop.  See also the while and continue
              requests.  Be sure not to confuse this with the br request.

       .brp   This is the same as \p.

       .cflags n c1 c2 ...
              Characters c1, c2, ..., have properties determined by n, which
              is ORed from the following:

              1      The character ends sentences (initially characters .?!
                     have this property).

              2      Lines can be broken before the character (initially no
                     characters have this property); a line is not broken at a
                     character with this property unless the characters on
                     each side both have non-zero hyphenation codes.  This can
                     be overridden with value 64.

              4      Lines can be broken after the character (initially
                     characters -\[hy]\[em] have this property); a line is not
                     broken at a character with this property unless the
                     characters on each side both have non-zero hyphenation
                     codes.  This can be overridden with value 64.

              8      The glyph associated with this character overlaps
                     horizontally (initially characters
                     \[ul]\[rn]\[ru]\[radicalex]\[sqrtex] have this property).

              16     The glyph associated with this character overlaps
                     vertically (initially glyph \[br] has this property).

              32     An end-of-sentence character followed by any number of
                     characters with this property is treated as the end of a
                     sentence if followed by a newline or two spaces; in other
                     words the character is transparent for the purposes of
                     end-of-sentence recognition; this is the same as having a
                     zero space factor in TeX (initially characters
                     "')]*\[dg]\[rq] have this property).

              64     Ignore hyphenation code values of the surrounding
                     characters.  Use this in combination with values 2 and 4
                     (initially no characters have this property).

              128    Prohibit a line break before the character, but allow a
                     line break after the character.  This works only in
                     combination with flags 256 and 512 and has no effect
                     otherwise.

              256    Prohibit a line break after the character, but allow a
                     line break before the character.  This works only in
                     combination with flags 128 and 512 and has no effect
                     otherwise.

              512    Allow line break before or after the character.  This
                     works only in combination with flags 128 and 256 and has
                     no effect otherwise.

              Contrary to flag values 2 and 4, the flags 128, 256, and 512
              work pairwise.  If, for example, the left character has value
              512, and the right character 128, no line break gets inserted.
              If we use value 6 instead for the left character, a line break
              after the character can't be suppressed since the right
              neighbour character doesn't get examined.

       .char c string
              [This request can both define characters and glyphs.]

              Define entity c to be string.  To be more precise, define (or
              even override) a groff entity which can be accessed with name c
              on the input side, and which uses string on the output side.
              Every time glyph c needs to be printed, string is processed in a
              temporary environment and the result is wrapped up into a single
              object.  Compatibility mode is turned off and the escape
              character is set to \ while string is being processed.  Any
              emboldening, constant spacing or track kerning is applied to
              this object rather than to individual glyphs in string.

              A groff object defined by this request can be used just like a
              normal glyph provided by the output device.  In particular other
              characters can be translated to it with the tr request; it can
              be made the leader glyph by the lc request; repeated patterns
              can be drawn with the glyph using the \l and \L escape
              sequences; words containing c can be hyphenated correctly, if
              the hcode request is used to give the object a hyphenation code.

              There is a special anti-recursion feature: Use of glyph within
              the glyph's definition is handled like normal glyphs not defined
              with char.

              A glyph definition can be removed with the rchar request.

       .chop xx
              Chop the last element off macro, string, or diversion xx.  This
              is useful for removing the newline from the end of diversions
              that are to be interpolated as strings.

       .class name c1 c2 ...
              Assign name to a set of characters c1, c2, ..., so that they can
              be referred to from other requests easily (currently .cflags
              only).  Character ranges (indicated by an intermediate `-') and
              nested classes are possible also.  This is useful to assign
              properties to a large set of characters.

       .close stream
              Close the stream named stream; stream will no longer be an
              acceptable argument to the write request.  See the open request.

       .composite glyph1 glyph2
              Map glyph name glyph1 to glyph name glyph2 if it is used in
              \[...]  with more than one component.

       .continue
              Finish the current iteration of a while loop.  See also the
              while and break requests.

       .color n
              If n is non-zero or missing, enable colors (this is the
              default), otherwise disable them.

       .cp n  If n is non-zero or missing, enable compatibility mode,
              otherwise disable it.  In compatibility mode, long names are not
              recognized, and the incompatibilities caused by long names do
              not arise.

       .defcolor xxx scheme color_components
              Define color xxx.  scheme can be one of the following values:
              rgb (three components), cmy (three components), cmyk (four
              components), and gray or grey (one component).  Color components
              can be given either as a hexadecimal string or as positive
              decimal integers in the range 0-65535.  A hexadecimal string
              contains all color components concatenated; it must start with
              either # or ##.  The former specifies hex values in the range
              0-255 (which are internally multiplied by 257), the latter in
              the range 0-65535.  Examples: #FFC0CB (pink), ##ffff0000ffff
              (magenta).  A new scaling indicator f has been introduced which
              multiplies its value by 65536; this makes it convenient to
              specify color components as fractions in the range 0 to 1.
              Example:

                     .defcolor darkgreen rgb 0.1f 0.5f 0.2f

              Note that f is the default scaling indicator for the defcolor
              request, thus the above statement is equivalent to

                     .defcolor darkgreen rgb 0.1 0.5 0.2

              The color named default (which is device-specific) can't be
              redefined.  It is possible that the default color for \M and \m
              is not the same.

       .de1 xx yy
              Similar to .de, but compatibility mode is switched off during
              execution.  On entry, the current compatibility mode is saved
              and restored at exit.

       .dei xx yy
              Define macro indirectly.  The following example

                     .ds xx aa
                     .ds yy bb
                     .dei xx yy

              is equivalent to

                     .de aa bb

       .dei1 xx yy
              Similar to the dei request but compatibility mode is switched
              off during execution.

       .device anything
              This is (almost) the same as the \X escape.  anything is read in
              copy mode; a leading " is stripped.

       .devicem xx
              This is the same as the \Y escape (to embed the contents of a
              macro into the intermediate output preceded with `x X').

       .do xxx
              Interpret .xxx with compatibility mode disabled.  For example,

                     .do fam T

              would have the same effect as

                     .fam T

              except that it would work even if compatibility mode had been
              enabled.  Note that the previous compatibility mode is restored
              before any files sourced by xxx are interpreted.

       .ds1 xx yy
              Similar to .ds, but compatibility mode is switched off during
              expansion.  To be more precise, a `compatibility save' token is
              inserted at the beginning of the string, and a `compatibility
              restore' token at the end.

       .ecs   Save current escape character.

       .ecr   Restore escape character saved with ecs.  Without a previous
              call to ecs, `\' will be the new escape character.

       .evc xx
              Copy the contents of environment xx to the current environment.
              No pushing or popping of environments is done.

       .fam xx
              Set the current font family to xx.  The current font family is
              part of the current environment.  If xx is missing, switch back
              to previous font family.  The value at start-up is `T'.  See the
              description of the sty request for more information on font
              families.

       .fchar c string
              Define fallback character (or glyph) c to be string.  The syntax
              of this request is the same as the char request; the only
              difference is that a glyph defined with char hides the glyph
              with the same name in the current font, whereas a glyph defined
              with fchar is checked only if the particular glyph isn't found
              in the current font.  This test happens before checking special
              fonts.

       .fcolor c
              Set the fill color to c.  If c is missing, switch to the
              previous fill color.

       .fschar f c string
              Define fallback character (or glyph) c for font f to be string.
              The syntax of this request is the same as the char request (with
              an additional argument to specify the font); a glyph defined
              with fschar is searched after the list of fonts declared with
              the fspecial request but before the list of fonts declared with
              .special.

       .fspecial f s1 s2 ...
              When the current font is f, fonts s1, s2, ..., are special, that
              is, they are searched for glyphs not in the current font.  Any
              fonts specified in the special request are searched after fonts
              specified in the fspecial request.  Without argument, reset the
              list of global special fonts to be empty.

       .ftr f g
              Translate font f to g.  Whenever a font named f is referred to
              in an \f escape sequence, in the F and S conditional operators,
              or in the ft, ul, bd, cs, tkf, special, fspecial, fp, or sty
              requests, font g is used.  If g is missing, or equal to f then
              font f is not translated.

       .fzoom f zoom
              Set zoom factor zoom for font f.  zoom must a non-negative
              integer multiple of 1/1000th.  If it is missing or is equal to
              zero, it means the same as 1000, namely no magnification.
              f must be a real font name, not a style.

       .gcolor c
              Set the glyph color to c.  If c is missing, switch to the
              previous glyph color.

       .hcode c1 code1 c2 code2 ...
              Set the hyphenation code of character c1 to code1 and that of c2
              to code2, and so on.  A hyphenation code must be a single input
              character (not a special character) other than a digit or a
              space.  Initially each lower-case letter a-z has a hyphenation
              code, which is itself, and each upper-case letter A-Z has a
              hyphenation code which is the lower-case version of itself.  See
              also the hpf request.

       .hla lang
              Set the current hyphenation language to lang.  Hyphenation
              exceptions specified with the hw request and hyphenation
              patterns specified with the hpf request are both associated with
              the current hyphenation language.  The hla request is usually
              invoked by the troffrc file to set up a default language.

       .hlm n Set the maximum number of consecutive hyphenated lines to n.  If
              n is negative, there is no maximum.  The default value is -1.
              This value is associated with the current environment.  Only
              lines output from an environment count towards the maximum
              associated with that environment.  Hyphens resulting from \% are
              counted; explicit hyphens are not.

       .hpf file
              Read hyphenation patterns from file; this is searched for in the
              same way that name.tmac is searched for when the -mname option
              is specified.  It should have the same format as (simple) TeX
              patterns files.  More specifically, the following scanning rules
              are implemented.

              o      A percent sign starts a comment (up to the end of the
                     line) even if preceded by a backslash.

              o      No support for `digraphs' like \$.

              o      ^^xx (x is 0-9 or a-f) and ^^x (character code of x in
                     the range 0-127) are recognized; other use of ^ causes an
                     error.

              o      No macro expansion.

              o      hpf checks for the expression \patterns{...} (possibly
                     with whitespace before and after the braces).  Everything
                     between the braces is taken as hyphenation patterns.
                     Consequently, { and } are not allowed in patterns.

              o      Similarly, \hyphenation{...} gives a list of hyphenation
                     exceptions.

              o      \endinput is recognized also.

              o      For backwards compatibility, if \patterns is missing, the
                     whole file is treated as a list of hyphenation patterns
                     (only recognizing the % character as the start of a
                     comment).

              Use the hpfcode request to map the encoding used in hyphenation
              patterns files to groff's input encoding.  By default,
              everything maps to itself except letters `A' to `Z' which map to
              `a' to `z'.

              The set of hyphenation patterns is associated with the current
              language set by the hla request.  The hpf request is usually
              invoked by the troffrc file; a second call replaces the old
              patterns with the new ones.

       .hpfa file
              The same as hpf except that the hyphenation patterns from file
              are appended to the patterns already loaded in the current
              language.

       .hpfcode a b c d ...
              After reading a hyphenation patterns file with the hpf or hpfa
              request, convert all characters with character code a in the
              recently read patterns to character code b, character code c
              to d, etc.  Initially, all character codes map to themselves.
              The arguments of hpfcode must be integers in the range 0 to 255.
              Note that it is even possible to use character codes which are
              invalid in groff otherwise.

       .hym n Set the hyphenation margin to n: when the current adjustment
              mode is not b, the line is not hyphenated if the line is no more
              than n short.  The default hyphenation margin is 0.  The default
              scaling indicator for this request is m.  The hyphenation margin
              is associated with the current environment.  The current
              hyphenation margin is available in the \n[.hym] register.

       .hys n Set the hyphenation space to n: When the current adjustment mode
              is b don't hyphenate the line if the line can be justified by
              adding no more than n extra space to each word space.  The
              default hyphenation space is 0.  The default scaling indicator
              for this request is m.  The hyphenation space is associated with
              the current environment.  The current hyphenation space is
              available in the \n[.hys] register.

       .itc n macro
              Variant of .it for which a line interrupted with \c counts as
              one input line.

       .kern n
              If n is non-zero or missing, enable pairwise kerning, otherwise
              disable it.

       .length xx string
              Compute the length of string and return it in the number
              register xx (which is not necessarily defined before).

       .linetabs n
              If n is non-zero or missing, enable line-tabs mode, otherwise
              disable it (which is the default).  In line-tabs mode, tab
              distances are computed relative to the (current) output line.
              Otherwise they are taken relative to the input line.  For
              example, the following

                     .ds x a\t\c
                     .ds y b\t\c
                     .ds z c
                     .ta 1i 3i
                     \*x
                     \*y
                     \*z

              yields

                     a         b         c

              In line-tabs mode, the same code gives

                     a         b                   c

              Line-tabs mode is associated with the current environment; the
              read-only number register \n[.linetabs] is set to 1 if in line-
              tabs mode, and 0 otherwise.

       .lsm xx
              Set the leading spaces macro to xx.  If there are leading spaces
              in an input line, it is invoked instead of the usual troff
              behaviour; the leading spaces are removed.  Registers \n[lsn]
              and \n[lss] hold the number of removed leading spaces and the
              corresponding horizontal space, respectively.

       .mso file
              The same as the so request except that file is searched for in
              the same directories as macro files for the the -m command line
              option.  If the file name to be included has the form name.tmac
              and it isn't found, mso tries to include tmac.name instead and
              vice versa.  A warning of type file is generated if file can't
              be loaded, and the request is ignored.

       .nop anything
              Execute anything.  This is similar to `.if 1'.

       .nroff Make the n built-in condition true and the t built-in condition
              false.  This can be reversed using the troff request.

       .open stream filename
              Open filename for writing and associate the stream named stream
              with it.  See also the close and write requests.

       .opena stream filename
              Like open, but if filename exists, append to it instead of
              truncating it.

       .output string
              Emit string directly to the intermediate output (subject to
              copy-mode interpretation); this is similar to \! used at the top
              level.  An initial double quote in string is stripped off to
              allow initial blanks.

       .pev   Print the current environment and each defined environment state
              on stderr.

       .pnr   Print the names and contents of all currently defined number
              registers on stderr.

       .psbb filename
              Get the bounding box of a PostScript image filename.  This file
              must conform to Adobe's Document Structuring Conventions; the
              command looks for a %%BoundingBox comment to extract the
              bounding box values.  After a successful call, the coordinates
              (in PostScript units) of the lower left and upper right corner
              can be found in the registers \n[llx], \n[lly], \n[urx], and
              \n[ury], respectively.  If some error has occurred, the four
              registers are set to zero.

       .pso command
              This behaves like the so request except that input comes from
              the standard output of command.

       .ptr   Print the names and positions of all traps (not including input
              line traps and diversion traps) on stderr.  Empty slots in the
              page trap list are printed as well, because they can affect the
              priority of subsequently planted traps.

       .pvs +-n
              Set the post-vertical line space to n; default scale indicator
              is p.  This value is added to each line after it has been
              output.  With no argument, the post-vertical line space is set
              to its previous value.

              The total vertical line spacing consists of four components: .vs
              and \x with a negative value which are applied before the line
              is output, and .pvs and \x with a positive value which are
              applied after the line is output.

       .rchar c1 c2 ...
              Remove the definitions of glyphs c1, c2, ...  This undoes the
              effect of a char request.

       .return
              Within a macro, return immediately.  If called with an argument,
              return twice, namely from the current macro and from the macro
              one level higher.  No effect otherwise.

       .rfschar c1 c2 ...
              Remove the font-specific definitions of glyphs c1, c2, ...  This
              undoes the effect of a fschar request.

       .rj
       .rj n  Right justify the next n input lines.  Without an argument right
              justify the next input line.  The number of lines to be right
              justified is available in the \n[.rj] register.  This implicitly
              does .ce 0.  The ce request implicitly does .rj 0.

       .rnn xx yy
              Rename number register xx to yy.

       .schar c string
              Define global fallback character (or glyph) c to be string.  The
              syntax of this request is the same as the char request; a glyph
              defined with schar is searched after the list of fonts declared
              with the special request but before the mounted special fonts.

       .shc c Set the soft hyphen character to c.  If c is omitted, the soft
              hyphen character is set to the default \[hy].  The soft hyphen
              character is the glyph which is inserted when a word is
              hyphenated at a line break.  If the soft hyphen character does
              not exist in the font of the glyph immediately preceding a
              potential break point, then the line is not broken at that
              point.  Neither definitions (specified with the char request)
              nor translations (specified with the tr request) are considered
              when finding the soft hyphen character.

       .shift n
              In a macro, shift the arguments by n positions: argument i
              becomes argument i-n; arguments 1 to n are no longer available.
              If n is missing, arguments are shifted by 1.  Shifting by
              negative amounts is currently undefined.

       .sizes s1 s2 ... sn [0]
              This command is similar to the sizes command of a DESC file.  It
              sets the available font sizes for the current font to s1, s2,
              ..., sn scaled points.  The list of sizes can be terminated by
              an optional 0.  Each si can also be a range of sizes m-n.
              Contrary to the font file command, the list can't extend over
              more than a single line.

       .special s1 s2 ...
              Fonts s1, s2, ..., are special and are searched for glyphs not
              in the current font.  Without arguments, reset the list of
              special fonts to be empty.

       .spreadwarn limit
              Make troff emit a warning if the additional space inserted for
              each space between words in an output line is larger or equal to
              limit.  A negative value is changed to zero; no argument toggles
              the warning on and off without changing limit.  The default
              scaling indicator is m.  At startup, spreadwarn is deactivated,
              and limit is set to 3m.  For example, .spreadwarn 0.2m causes a
              warning if troff must add 0.2m or more for each interword space
              in a line.  This request is active only if text is justified to
              both margins (using .ad b).

       .sty n f
              Associate style f with font position n.  A font position can be
              associated either with a font or with a style.  The current font
              is the index of a font position and so is also either a font or
              a style.  When it is a style, the font that is actually used is
              the font the name of which is the concatenation of the name of
              the current family and the name of the current style.  For
              example, if the current font is 1 and font position 1 is
              associated with style R and the current font family is T, then
              font TR is used.  If the current font is not a style, then the
              current family is ignored.  When the requests cs, bd, tkf, uf,
              or fspecial are applied to a style, then they are applied
              instead to the member of the current family corresponding to
              that style.  The default family can be set with the -f command
              line option.  The styles command in the DESC file controls which
              font positions (if any) are initially associated with styles
              rather than fonts.

       .substring xx n1 [n2]
              Replace the string named xx with the substring defined by the
              indices n1 and n2.  The first character in the string has
              index 0.  If n2 is omitted, it is taken to be equal to the
              string's length.  If the index value n1 or n2 is negative, it is
              counted from the end of the string, going backwards: The last
              character has index -1, the character before the last character
              has index -2, etc.

       .tkf f s1 n1 s2 n2
              Enable track kerning for font f.  When the current font is f the
              width of every glyph is increased by an amount between n1 and
              n2; when the current point size is less than or equal to s1 the
              width is increased by n1; when it is greater than or equal to s2
              the width is increased by n2; when the point size is greater
              than or equal to s1 and less than or equal to s2 the increase in
              width is a linear function of the point size.

       .tm1 string
              Similar to the tm request, string is read in copy mode and
              written on the standard error, but an initial double quote in
              string is stripped off to allow initial blanks.

       .tmc string
              Similar to tm1 but without writing a final newline.

       .trf filename
              Transparently output the contents of file filename.  Each line
              is output as if preceded by \!; however, the lines are not
              subject to copy-mode interpretation.  If the file does not end
              with a newline, then a newline is added.  For example, you can
              define a macro x containing the contents of file f, using

                     .di x
                     .trf f
                     .di

              Unlike with the cf request, the file cannot contain characters
              such as NUL that are not valid troff input characters.

       .trin abcd
              This is the same as the tr request except that the asciify
              request uses the character code (if any) before the character
              translation.  Example:

                     .trin ax
                     .di xxx
                     a
                     .br
                     .di
                     .xxx
                     .trin aa
                     .asciify xxx
                     .xxx

              The result is x a.  Using tr, the result would be x x.

       .trnt abcd
              This is the same as the tr request except that the translations
              do not apply to text that is transparently throughput into a
              diversion with \!.  For example,

                     .tr ab
                     .di x
                     \!.tm a
                     .di
                     .x

              prints b; if trnt is used instead of tr it prints a.

       .troff Make the n built-in condition false, and the t built-in
              condition true.  This undoes the effect of the nroff request.

       .unformat xx
              This request `unformats' the diversion xx.  Contrary to the
              asciify request, which tries to convert formatted elements of
              the diversion back to input tokens as much as possible,
              .unformat only handles tabs and spaces between words (usually
              caused by spaces or newlines in the input) specially.  The
              former are treated as if they were input tokens, and the latter
              are stretchable again.  Note that the vertical size of lines is
              not preserved.  Glyph information (font, font size, space width,
              etc.) is retained.  Useful in conjunction with the box and boxa
              requests.

       .vpt n Enable vertical position traps if n is non-zero, disable them
              otherwise.  Vertical position traps are traps set by the wh or
              dt requests.  Traps set by the it request are not vertical
              position traps.  The parameter that controls whether vertical
              position traps are enabled is global.  Initially vertical
              position traps are enabled.

       .warn n
              Control warnings.  n is the sum of the numbers associated with
              each warning that is to be enabled; all other warnings are
              disabled.  The number associated with each warning is listed in
              troff(1).  For example, .warn 0 disables all warnings, and
              .warn 1 disables all warnings except that about missing glyphs.
              If n is not given, all warnings are enabled.

       .warnscale si
              Set the scaling indicator used in warnings to si.  Valid values
              for si are u, i, c, p, and P.  At startup, it is set to i.

       .while c anything
              While condition c is true, accept anything as input; c can be
              any condition acceptable to an if request; anything can comprise
              multiple lines if the first line starts with \{ and the last
              line ends with \}.  See also the break and continue requests.

       .write stream anything
              Write anything to the stream named stream.  stream must
              previously have been the subject of an open request.  anything
              is read in copy mode; a leading " is stripped.

       .writec stream anything
              Similar to write but without writing a final newline.

       .writem stream xx
              Write the contents of the macro or string xx to the stream named
              stream.  stream must previously have been the subject of an open
              request.  xx is read in copy mode.

   Extended escape sequences
       \D'...'
              All drawing commands of groff's intermediate output are
              accepted.  See subsection Drawing Commands below for more
              information.

   Extended requests
       .cf filename
              When used in a diversion, this embeds in the diversion an object
              which, when reread, will cause the contents of filename to be
              transparently copied through to the output.  In UNIX troff, the
              contents of filename is immediately copied through to the output
              regardless of whether there is a current diversion; this
              behaviour is so anomalous that it must be considered a bug.

       .de xx yy
       .am xx yy
       .ds xx yy
       .as xx yy
              In compatibility mode, these requests behaves similar to .de1,
              .am1, .ds1, and .as1, respectively: A `compatibility save' token
              is inserted at the beginning, and a `compatibility restore'
              token at the end, with compatibility mode switched on during
              execution.

       .ev xx If xx is not a number, this switches to a named environment
              called xx.  The environment should be popped with a matching ev
              request without any arguments, just as for numbered
              environments.  There is no limit on the number of named
              environments; they are created the first time that they are
              referenced.

       .ss m n
              When two arguments are given to the ss request, the second
              argument gives the sentence space size.  If the second argument
              is not given, the sentence space size is the same as the word
              space size.  Like the word space size, the sentence space is in
              units of one twelfth of the spacewidth parameter for the current
              font.  Initially both the word space size and the sentence space
              size are 12.  Contrary to UNIX troff, GNU troff handles this
              request in nroff mode also; a given value is then rounded down
              to the nearest multiple of 12.  The sentence space size is used
              in two circumstances.  If the end of a sentence occurs at the
              end of a line in fill mode, then both an inter-word space and a
              sentence space are added; if two spaces follow the end of a
              sentence in the middle of a line, then the second space is a
              sentence space.  Note that the behaviour of UNIX troff are
              exactly that exhibited by GNU troff if a second argument is
              never given to the ss request.  In GNU troff, as in UNIX troff,
              you should always follow a sentence with either a newline or two
              spaces.

       .ta n1 n2 ... nn T r1 r2 ... rn
              Set tabs at positions n1, n2, ..., nn and then set tabs at
              nn+r1, nn+r2, ..., nn+rn and then at nn+rn+r1, nn+rn+r2, ...,
              nn+rn+rn, and so on.  For example,

                     .ta T .5i

              sets tabs every half an inch.

   New number registers
       The following read-only registers are available:

       \n[.br]
              Within a macro call, it is set to 1 if the macro is called with
              the `normal' control character (`.' by default), and set to 0
              otherwise.  This allows to reliably modify requests.

                     .als bp*orig bp
                     .de bp
                     .tm before bp
                     .ie \\n[.br] .bp*orig
                     .el 'bp*orig
                     .tm after bp
                     ..

              Using this register outside of a macro makes no sense (it always
              returns zero in such cases).

       \n[.C] 1 if compatibility mode is in effect, 0 otherwise.

       \n[.cdp]
              The depth of the last glyph added to the current environment.
              It is positive if the glyph extends below the baseline.

       \n[.ce]
              The number of lines remaining to be centered, as set by the ce
              request.

       \n[.cht]
              The height of the last glyph added to the current environment.
              It is positive if the glyph extends above the baseline.

       \n[.color]
              1 if colors are enabled, 0 otherwise.

       \n[.csk]
              The skew of the last glyph added to the current environment.
              The skew of a glyph is how far to the right of the center of a
              glyph the center of an accent over that glyph should be placed.

       \n[.ev]
              The name or number of the current environment.  This is a
              string-valued register.

       \n[.fam]
              The current font family.  This is a string-valued register.

       \n[.fn]
              The current (internal) real font name.  This is a string-valued
              register.  If the current font is a style, the value of \n[.fn]
              is the proper concatenation of family and style name.

       \n[.fp]
              The number of the next free font position.

       \n[.g] Always 1.  Macros should use this to determine whether they are
              running under GNU troff.

       \n[.height]
              The current height of the font as set with \H.

       \n[.hla]
              The current hyphenation language as set by the hla request.

       \n[.hlc]
              The number of immediately preceding consecutive hyphenated
              lines.

       \n[.hlm]
              The maximum allowed number of consecutive hyphenated lines, as
              set by the hlm request.

       \n[.hy]
              The current hyphenation flags (as set by the hy request).

       \n[.hym]
              The current hyphenation margin (as set by the hym request).

       \n[.hys]
              The current hyphenation space (as set by the hys request).

       \n[.in]
              The indentation that applies to the current output line.

       \n[.int]
              Set to a positive value if last output line is interrupted
              (i.e., if it contains \c).

       \n[.kern]
              1 if pairwise kerning is enabled, 0 otherwise.

       \n[.lg]
              The current ligature mode (as set by the lg request).

       \n[.linetabs]
              The current line-tabs mode (as set by the linetabs request).

       \n[.ll]
              The line length that applies to the current output line.

       \n[.lt]
              The title length as set by the lt request.

       \n[.m] The name of the current drawing color.  This is a string-valued
              register.

       \n[.M] The name of the current background color.  This is a string-
              valued register.

       \n[.ne]
              The amount of space that was needed in the last ne request that
              caused a trap to be sprung.  Useful in conjunction with the
              \n[.trunc] register.

       \n[.ns]
              1 if no-space mode is active, 0 otherwise.

       \n[.O] The current output level as set with \O.

       \n[.P] 1 if the current page is in the output list set with -o.

       \n[.pe]
              1 during a page ejection caused by the bp request, 0 otherwise.

       \n[.pn]
              The number of the next page, either the value set by a pn
              request, or the number of the current page plus 1.

       \n[.ps]
              The current point size in scaled points.

       \n[.psr]
              The last-requested point size in scaled points.

       \n[.pvs]
              The current post-vertical line space as set with the pvs
              request.

       \n[.rj]
              The number of lines to be right-justified as set by the rj
              request.

       \n[.slant]
              The slant of the current font as set with \S.

       \n[.sr]
              The last requested point size in points as a decimal fraction.
              This is a string-valued register.

       \n[.ss]
       \n[.sss]
              These give the values of the parameters set by the first and
              second arguments of the ss request.

       \n[.sty]
              The current font style.  This is a string-valued register.

       \n[.tabs]
              A string representation of the current tab settings suitable for
              use as an argument to the ta request.

       \n[.trunc]
              The amount of vertical space truncated by the most recently
              sprung vertical position trap, or, if the trap was sprung by a
              ne request, minus the amount of vertical motion produced by the
              ne request.  In  other  words, at the point  a  trap is  sprung,
              it represents the difference of  what the vertical position
              would have been but for the trap, and what the vertical position
              actually is.  Useful in conjunction with the \n[.ne] register.

       \n[.U] Set to 1 if in safer mode and to 0 if in unsafe mode (as given
              with the -U command line option).

       \n[.vpt]
              1 if vertical position traps are enabled, 0 otherwise.

       \n[.warn]
              The sum of the numbers associated with each of the currently
              enabled warnings.  The number associated with each warning is
              listed in troff(1).

       \n[.x] The major version number.  For example, if the version number is
              1.03, then \n[.x] contains 1.

       \n[.y] The minor version number.  For example, if the version number is
              1.03, then \n[.y] contains 03.

       \n[.Y] The revision number of groff.

       \n[.zoom]
              The zoom value of the current font, in multiples of 1/1000th.
              Zero if no magnification.

       \n[llx]
       \n[lly]
       \n[urx]
       \n[ury]
              These four read/write registers are set by the psbb request and
              contain the bounding box values (in PostScript units) of a given
              PostScript image.

       The following read/write registers are set by the \w escape sequence:

       \n[rst]
       \n[rsb]
              Like the st and sb registers, but take account of the heights
              and depths of glyphs.

       \n[ssc]
              The amount of horizontal space (possibly negative) that should
              be added to the last glyph before a subscript.

       \n[skw]
              How far to right of the center of the last glyph in the \w
              argument, the center of an accent from a roman font should be
              placed over that glyph.

       Other available read/write number registers are:

       \n[c.] The current input line number.  \n[.c] is a read-only alias to
              this register.

       \n[hours]
              The number of hours past midnight.  Initialized at start-up.

       \n[hp] The current horizontal position at input line.

       \n[lsn]
       \n[lss]
              If there are leading spaces in an input line, these registers
              hold the number of leading spaces and the corresponding
              horizontal space, respectively.

       \n[minutes]
              The number of minutes after the hour.  Initialized at start-up.

       \n[seconds]
              The number of seconds after the minute.  Initialized at start-
              up.

       \n[systat]
              The return value of the system() function executed by the last
              sy request.

       \n[slimit]
              If greater than 0, the maximum number of objects on the input
              stack.  If less than or equal to 0, there is no limit on the
              number of objects on the input stack.  With no limit, recursion
              can continue until virtual memory is exhausted.

       \n[year]
              The current year.  Note that the traditional troff number
              register \n[yr] is the current year minus 1900.

   Miscellaneous
       troff predefines a single (read/write) string-based register, \*[.T],
       which contains the argument given to the -T command line option, namely
       the current output device (for example, latin1 or ascii).  Note that
       this is not the same as the (read-only) number register \n[.T] which is
       defined to be 1 if troff is called with the -T command line option, and
       zero otherwise.  This behaviour is different to UNIX troff.

       Fonts not listed in the DESC file are automatically mounted on the next
       available font position when they are referenced.  If a font is to be
       mounted explicitly with the fp request on an unused font position, it
       should be mounted on the first unused font position, which can be found
       in the \n[.fp] register; although troff does not enforce this strictly,
       it does not allow a font to be mounted at a position whose number is
       much greater than that of any currently used position.

       Interpolating a string does not hide existing macro arguments.  Thus in
       a macro, a more efficient way of doing

              .xx \\$@

       is

              \\*[xx]\\

       If the font description file contains pairwise kerning information,
       glyphs from that font are kerned.  Kerning between two glyphs can be
       inhibited by placing a \& between them.

       In a string comparison in a condition, characters that appear at
       different input levels to the first delimiter character are not
       recognized as the second or third delimiters.  This applies also to the
       tl request.  In a \w escape sequence, a character that appears at a
       different input level to the starting delimiter character is not
       recognized as the closing delimiter character.  The same is true for
       \A, \b, \B, \C, \l, \L, \o, \X, and \Z.  When decoding a macro or
       string argument that is delimited by double quotes, a character that
       appears at a different input level to the starting delimiter character
       is not recognized as the closing delimiter character.  The
       implementation of \$@ ensures that the double quotes surrounding an
       argument appear at the same input level, which is different to the
       input level of the argument itself.  In a long escape name ] is not
       recognized as a closing delimiter except when it occurs at the same
       input level as the opening ].  In compatibility mode, no attention is
       paid to the input-level.

       There are some new types of condition:

       .if rxxx
              True if there is a number register named xxx.

       .if dxxx
              True if there is a string, macro, diversion, or request named
              xxx.

       .if mxxx
              True if there is a color named xxx.

       .if cch
              True if there is a character (or glyph) ch available; ch is
              either an ASCII character or a glyph (special character)
              \N'xxx', \(xx or \[xxx]; the condition is also true if ch has
              been defined by the char request.

       .if Ff True if font f exists.  f is handled as if it was opened with
              the ft request (this is, font translation and styles are
              applied), without actually mounting it.

       .if Ss True if style s has been registered.  Font translation is
              applied.

       The tr request can now map characters onto \~.

       The space width emitted by the \| and \^ escape sequences can be
       controlled on a per-font basis.  If there is a glyph named \| or \^,
       respectively (note the leading backslash), defined in the current font
       file, use this glyph's width instead of the default value.

       It is now possible to have whitespace between the first and second dot
       (or the name of the ending macro) to end a macro definition.  Example:

              .if t \{\
              .  de bar
              .    nop Hello, I'm `bar'.
              .  .
              .\}

INTERMEDIATE OUTPUT FORMAT
       This section describes the format output by GNU troff.  The output
       format used by GNU troff is very similar to that used by Unix device-
       independent troff.  Only the differences are documented here.

   Units
       The argument to the s command is in scaled points (units of points/n,
       where n is the argument to the sizescale command  in the DESC file).
       The argument to the x Height command is also in scaled points.

   Text Commands
       Nn     Print glyph with index n (a non-negative integer) of the current
              font.

       If the tcommand line is present in the DESC file, troff uses the
       following two commands.

       txxx   xxx is any sequence of characters terminated by a space or a
              newline (to be more precise, it is a sequence of glyphs which
              are accessed with the corresponding characters); the first
              character should be printed at the current position, the current
              horizontal position should be increased by the width of the
              first character, and so on for each character.  The width of the
              glyph is that given in the font file, appropriately scaled for
              the current point size, and rounded so that it is a multiple of
              the horizontal resolution.  Special characters cannot be printed
              using this command.

       un xxx This is same as the t command except that after printing each
              character, the current horizontal position is increased by the
              sum of the width of that character and n.

       Note that single characters can have the eighth bit set, as can the
       names of fonts and special characters.

       The names of glyphs and fonts can be of arbitrary length; drivers
       should not assume that they are only two characters long.

       When a glyph is to be printed, that glyph is always in the current
       font.  Unlike device-independent troff, it is not necessary for drivers
       to search special fonts to find a glyph.

       For color support, some new commands have been added:

       mc cyan magenta yellow
       md
       mg gray
       mk cyan magenta yellow black
       mr red green blue
              Set the color components of the current drawing color, using
              various color schemes.  md resets the drawing color to the
              default value.  The arguments are integers in the range 0 to
              65536.

       The x device control command has been extended.

       x u n  If n is 1, start underlining of spaces.  If n is 0, stop
              underlining of spaces.  This is needed for the cu request in
              nroff mode and is ignored otherwise.

   Drawing Commands
       The D drawing command has been extended.  These extensions are not used
       by GNU pic if the -n option is given.

       Df n\n Set the shade of gray to be used for filling solid objects to n;
              n must be an integer between 0 and 1000, where 0 corresponds
              solid white and 1000 to solid black, and values in between
              correspond to intermediate shades of gray.  This applies only to
              solid circles, solid ellipses and solid polygons.  By default, a
              level of 1000 is used.  Whatever color a solid object has, it
              should completely obscure everything beneath it.  A value
              greater than 1000 or less than 0 can also be used: this means
              fill with the shade of gray that is currently being used for
              lines and text.  Normally this is black, but some drivers may
              provide a way of changing this.

              The corresponding \D'f...'  command shouldn't be used since its
              argument is always rounded to an integer multiple of the
              horizontal resolution which can lead to surprising results.

       DC d\n Draw a solid circle with a diameter of d with the leftmost point
              at the current position.

       DE dx dy\n
              Draw a solid ellipse with a horizontal diameter of dx and a
              vertical diameter of dy with the leftmost point at the current
              position.  delim $$

       Dp $dx sub 1$ $dy sub 1$ $dx sub 2$ $dy sub 2$ $...$ $dx sub n$ $dy sub
       n$\n
              Draw a polygon with, for $i = 1 ,..., n+1$, the i-th vertex at
              the current position $+ sum from j=1 to i-1 ( dx sub j , dy sub
              j )$.  At the moment, GNU pic only uses this command to generate
              triangles and rectangles.

       DP $dx sub 1$ $dy sub 1$ $dx sub 2$ $dy sub 2$ $...$ $dx sub n$ $dy sub
       n$\n
              Like Dp but draw a solid rather than outlined polygon.

       Dt n\n Set the current line thickness to n machine units.
              Traditionally Unix troff drivers use a line thickness
              proportional to the current point size; drivers should continue
              to do this if no Dt command has been given, or if a Dt command
              has been given with a negative value of n.  A zero value of n
              selects the smallest available line thickness.

       A difficulty arises in how the current position should be changed after
       the execution of these commands.  This is not of great importance since
       the code generated by GNU pic does not depend on this.  Given a drawing
       command of the form

              \D'c $x sub 1$ $y sub 1$ $x sub 2$ $y sub 2$ $...$ $x sub n$ $y
              sub n$'

       where c is not one of c, e, l, a, or ~, Unix troff treats each of the
       $x sub i$ as a horizontal quantity, and each of the $y sub i$ as a
       vertical quantity and assumes that the width of the drawn object is
       $sum from i=1 to n x sub i$, and that the height is $sum from i=1 to n
       y sub i$.  (The assumption about the height can be seen by examining
       the st and sb registers after using such a D command in a \w escape
       sequence).  This rule also holds for all the original drawing commands
       with the exception of De.  For the sake of compatibility GNU troff also
       follows this rule, even though it produces an ugly result in the case
       of the Dt and Df, and, to a lesser extent, DE commands.  Thus after
       executing a D command of the form

              Dc $x sub 1$ $y sub 1$ $x sub 2$ $y sub 2$ $...$ $x sub n$ $y
              sub n$\n

       the current position should be increased by $( sum from i=1 to n x sub
       i , sum from i=1 to n y sub i )$.

       Another set of extensions is

       DFc cyan magenta yellow\n
       DFd\n
       DFg gray\n
       DFk cyan magenta yellow black\n
       DFr red green blue\n
              Set the color components of the filling color similar to the
              m commands above.

       The current position isn't changed by those colour commands (contrary
       to Df).

   Device Control Commands
       There is a continuation convention which permits the argument to the
       x X command to contain newlines: when outputting the argument to the
       x X command, GNU troff follows each newline in the argument with a +
       character (as usual, it terminates the entire argument with a newline);
       thus if the line after the line containing the x X command starts with
       +, then the newline ending the line containing the x X command should
       be treated as part of the argument to the x X command, the + should be
       ignored, and the part of the line following the + should be treated
       like the part of the line following the x X command.

       The first three output commands are guaranteed to be:

              x T device
              x res n h v
              x init

INCOMPATIBILITIES
       In spite of the many extensions, groff has retained compatibility to
       classical troff to a large degree.  For the cases where the extensions
       lead to collisions, a special compatibility mode with the restricted,
       old functionality was created for groff.

   Groff Language
       groff provides a compatibility mode that allows to process roff code
       written for classical troff or for other implementations of roff in a
       consistent way.

       Compatibility mode can be turned on with the -C command line option,
       and turned on or off with the .cp request.  The number register \n(.C
       is 1 if compatibility mode is on, 0 otherwise.

       This became necessary because the GNU concept for long names causes
       some incompatibilities.  Classical troff interprets

              .dsabcd

       as defining a string ab with contents cd.  In groff mode, this is
       considered as a call of a macro named dsabcd.

       Also classical troff interprets \*[ or \n[ as references to a string or
       number register called [ while groff takes this as the start of a long
       name.

       In compatibility mode, groff interprets these things in the traditional
       way; so long names are not recognized.

       On the other hand, groff in GNU native mode does not allow to use the
       single-character escapes \\ (backslash), \| (vertical bar), \^ (caret),
       \& (ampersand), \{ (opening brace), \} (closing brace), `\ ' (space),
       \' (single quote), \` (backquote), \- (minus), \_ (underline), \!
       (bang), \% (percent), and \c (character c) in names of strings, macros,
       diversions, number registers, fonts or environments, whereas classical
       troff does.

       The \A escape sequence can be helpful in avoiding these escape
       sequences in names.

       Fractional point sizes cause one noteworthy incompatibility.  In
       classical troff, the ps request ignores scale indicators and so

              .ps 10u

       sets the point size to 10 points, whereas in groff native mode the
       point size is set to 10 scaled points.

       In groff, there is a fundamental difference between unformatted input
       characters, and formatted output characters (glyphs).  Everything that
       affects how a glyph is output is stored with the glyph; once a glyph
       has been constructed it is unaffected by any subsequent requests that
       are executed, including the bd, cs, tkf, tr, or fp requests.

       Normally glyphs are constructed from input characters at the moment
       immediately before the glyph is added to the current output line.
       Macros, diversions and strings are all, in fact, the same type of
       object; they contain lists of input characters and glyphs in any
       combination.

       Special characters can be both; before being added to the output, they
       act as input entities, afterwards they denote glyphs.

       A glyph does not behave like an input character for the purposes of
       macro processing; it does not inherit any of the special properties
       that the input character from which it was constructed might have had.
       The following example makes things clearer.

              .di x
              \\\\
              .br
              .di
              .x

       With GNU troff this is printed as \\.  So each pair of input
       backslashes `\\' is turned into a single output backslash glyph `\' and
       the resulting output backslashes are not interpreted as escape
       characters when they are reread.

       Classical troff would interpret them as escape characters when they
       were reread and would end up printing a single backslash `\'.

       In GNU, the correct way to get a printable version of the backslash
       character `\' is the \(rs escape sequence, but classical troff does not
       provide a clean feature for getting a non-syntactical backslash.  A
       close method is the printable version of the current escape character
       using the \e escape sequence; this works if the current escape
       character is not redefined.  It works in both GNU mode and
       compatibility mode, while dirty tricks like specifying a sequence of
       multiple backslashes do not work reliably; for the different handling
       in diversions, macro definitions, or text mode quickly leads to a
       confusion about the necessary number of backslashes.

       To store an escape sequence in a diversion that is interpreted when the
       diversion is reread, either the traditional \! transparent output
       facility or the new \? escape sequence can be used.

   Intermediate Output
       The groff intermediate output format is in a state of evolution.  So
       far it has some incompatibilities, but it is intended to establish a
       full compatibility to the classical troff output format.  Actually the
       following incompatibilities exist:

       o The positioning after the drawing of the polygons conflicts with the
         classical definition.

       o The intermediate output cannot be rescaled to other devices as
         classical `device-independent' troff did.

AUTHORS
       Copyright (C) 1989, 2001, 2002, 2003, 2004, 2006, 2007, 2008, 2009,
       2010 Free Software Foundation, Inc.

       This document is distributed under the terms of the FDL (GNU Free
       Documentation License) version 1.3 or later.  You should have received
       a copy of the FDL on your system, it is also available on-line at the
       GNU copyleft site <http://www.gnu.org/copyleft/fdl.html>.  This
       document was written by James Clark, with modifications by Werner
       Lemberg <wl@gnu.org> and Bernd Warken <bwarken@mayn.de>.

       This document is part of groff, the GNU roff distribution.  Formerly,
       the contents of this document was kept in the manual page troff(1).
       Only the parts dealing with the language aspects of the different roff
       systems were carried over into this document.  The troff command line
       options and warnings are still documented in troff(1).

SEE ALSO
       The groff info file, cf. info(1) presents all groff documentation
       within a single document.

       groff(1)
              A list of all documentation around groff.

       groff(7)
              A description of the groff language, including a short, but
              complete reference of all predefined requests, registers, and
              escapes of plain groff.  From the command line, this is called
              using

                     man 7 groff

       roff(7)
              A survey of roff systems, including pointers to further
              historical documentation.

       [CSTR #54]
              The Nroff/Troff User's Manual by J. F. Ossanna of 1976 in the
              revision of Brian Kernighan of 1992, being the classical troff
              documentation <http://cm.bell-labs.com/cm/cs/cstr/54.ps.gz>.



Groff Version 1.21             31 December 2010                  GROFF_DIFF(7)
