On Wed, 22 Dec 2010, John Cowan wrote:

> However, this is a problem not of hygiene but of reusing implementation,
> I think.  Consider this:
>
>        (define cdr #f)
>        (length '(a b c))
>
> I haven't found any Schemes except Scheme 9 on which this fails, but
> there is nothing in any Scheme standard to make it work.

R5RS chapter 6 intro:

A program may use a top-level definition to bind any variable. It may
subsequently alter any such binding by an assignment (see 4.1.6). These
operations DO NOT MODIFY the behavior of Scheme's built-in procedures.

Andre

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