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Allegro CL |
ANSI Common Lisp  2 Syntax 2.4 Standard Macro Characters 2.4.8 Sharpsign
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. |