Vertical alignment

The example demonstrates the effect of several text attributes for vertical
alignment.

In the first line of the output, the extent of the whole text box is taken into
account by various versions of valign. While in this case, the valign attributes
are combined with a parbox each, the valign technique also perfectly works for
single line text. ...

In contrast, in the second line of the output a parbox is shown with different
values for the baseline. Note that by default, text is vertically aligned at
its baseline, that is, the line the letters are typed upon. For a parbox
containing several lines, there are several choices. PyX allows you to use the
top-most baseline (which is, by the way, the default when skipping the baseline
keyword argument of the parbox completely), and the bottom-most baseline in
your paragraph. Additionally you can also use the position just in the middle
of these two cases. Since the alignment along a baseline does not depend on the
size of the characters in your output, it is preferable to valign in most
use-cases.

In the last line of the output, the text.vshift is used to create some vertical
shift of the output corresponding to certain criteria. The universal case is
to use a certain fraction of a character height (or the height of any TeX
expression). For `vshift.topzero` the full height of the character `0` is used,
for `vshift.middlezero` half of the height of the character `0` is used and for
`vshift.bottomzero`, no shift will be done at all and the usual alignment at the
baseline will take place. Another interesting vertical shift value is the
`vshift.mathaxis`, which shifts the material by the height of the mathematical
axis. This axis is used by TeX to vertically align the mathematical operators
and the equal sign but also brackets and objects like the sum and integral
symbols. This shift typically is a little less in height as compared to
`vshift.middlezero` and fits well in various cases to visually center a single
line output in vertical direction. It is also useful to align text, although
the value of the shift is based on the configuration of the mathematical mode.
This means that the alignment might not be suitable for cases, where the
text mode of TeX was reconfigured without also adapting the math mode configuration.

! Please don't get confused by the additional text.mathmode attribute in this
vshift example, which leads the text to be rendered in TeX's mathematical mode.
The `text.mathmode` attribute is by the way similar but not identical (due to the
use of `\displaystyle`) to encapsulating your expression in TeX's mathmode
delimiters `$`.
