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This is very timely, because I am currently working on a new release of
the SRFI, this time with a completely new implementation for sets and
bags. The integer-set implementation will follow in a later release.
Eqv? would be the relevant predicate, given that when you put something
into a standard Scheme data structure such as a pair or vector, what you
get out is guaranteed to be eqv? to what went in, per R5RS/R7RS 3.4.
I considered this approach, but I decided that it was overkill. Other
than set-adjoin(!), the only procedures that need such selection are the
set operations set-union(!) and set-intersection(!), and since they are
commutative, a simple rule of "first argument beats second argument"
So I am suggesting that set-adjoin(!) never replaces existing elements,
and adding a new operation set-replace(!) with three arguments: the set,
the element, and a default. If the set contains the element, it is
deleted and the element is adjoined (this is a no-op if they are eqv?);
if not, the default is inserted.