CLHS: Section 2.4.8.16

#

## 2.4.8.16 Sharpsign Sharpsign

`#`n`#`

`#`n`#`, where n is a required unsigned decimal integer, provides a reference to some object labeled by `#`n`=`; that is, `#`n`#` represents a pointer to the same (eq) object labeled by `#`n`=`. For example, a structure created in the variable `y` by this code:

     (setq x (list 'p 'q))
     (setq y (list (list 'a 'b) x 'foo x))
     (rplacd (last y) (cdr y))

could be represented in this way:

     ((a b) . #1=(#2=(p q) foo #2# . #1#))

Without this notation, but with *print-length* set to `10` and *print-circle* set to nil, the structure would print in this way:

     ((a b) (p q) foo (p q) (p q) foo (p q) (p q) foo (p q) ...)

A reference `#`n`#` may only occur after a label `#`n`=`; forward references are not permitted. The reference may not appear as the labeled object itself (that is, `#`n`=#`n`#`) may not be written because the object labeled by `#`n`=` is not well defined in this case.

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