In standard R[45]RS scheme (I don't know about R6RS), a programmer is not allowed to begin an identifier with ##; so identifiers beginning with ## are used internally, as a Gambit extension, to name procedures that the user is not supposed to redefine.

So the definition of cddr might be

(define (##cddr x) (##cdr (##cdr x))) (define (cddr x) (##cddr x))

so while the user can redefine cdr, that redefinition won't affect the definition of cddr, since cddr depends on ##cdr, not cdr.

Brad

On Dec 3, 2007, at 2:49 AM, Li Xiangyang wrote:

I see a lot of ## included in library file, such as ##define-macro, ##include, what does that mean? I can't find any explanation in documents.

thanks!

Howard

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