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(elisp)Circular Objects

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Read Syntax for Circular Objects
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   In Emacs 21, to represent shared or circular structure within a
complex of Lisp objects, you can use the reader constructs `#N=' and
`#N#'.

   Use `#N=' before an object to label it for later reference;
subsequently, you can use `#N#' to refer the same object in another
place.  Here, N is some integer.  For example, here is how to make a
list in which the first element recurs as the third element:

     (#1=(a) b #1#)

This differs from ordinary syntax such as this

     ((a) b (a))

which would result in a list whose first and third elements look alike
but are not the same Lisp object.  This shows the difference:

     (prog1 nil
       (setq x '(#1=(a) b #1#)))
     (eq (nth 0 x) (nth 2 x))
          => t
     (setq x '((a) b (a)))
     (eq (nth 0 x) (nth 2 x))
          => nil

   You can also use the same syntax to make a circular structure, which
appears as an "element" within itself.  Here is an example:

     #1=(a #1#)

This makes a list whose second element is the list itself.  Here's how
you can see that it really works:

     (prog1 nil
       (setq x '#1=(a #1#)))
     (eq x (cadr x))
          => t

   The Lisp printer can produce this syntax to record circular and
shared structure in a Lisp object, if you bind the variable
`print-circle' to a non-`nil' value.  Note: Output Variables.