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Re: DEL! and DELQ!
From: Seth Goldman <seth@UCLA-CS.ARPA>
These do not seem to work if the item to be deleted is non-atomic:
(set foo '(a b c)) ==> (a b c)
(delq! 'a foo) ==> (b c)
foo ==> (b c)
This is not a bug. The only contract of the foo! procedures (delq!,
reverse!, append!) is that they return a list of the correct form.
They are permitted to recycle the pairs in their argument to avoid
consing, if they want to, but the way in which they do this is not
prescribed. Besides, there is no way that a procedure can change
the assignment of a variable.