Kevin Wortman scripsit:

> I guess that's sufficient for my association set application; I can change
> a mapping by removing any pre-existing association then adjoining a new
> one. It's not ideal, though, since that is two data structure operations
> instead of just one.

That's precisely where you want to use `set-replace!`:

	(set-replace! set new-pair new-pair)

will remove any existing pair that is equal to new-pair in the sense
of the comparator (presumably one created with `make-car-comparator`),
and replaces it with new-pair; or if there is no such pair, adjoins
new-pair to the set.  It's tailor-made for your application.

--
John Cowan          http://www.ccil.org/~cowan        xxxxxx@ccil.org
What has four pairs of pants, lives in Philadelphia,
and it never rains but it pours?
        --Rufus T. Firefly