16. What provisions should we have for controlling which element of a
set is preserved when a new element equal to an existing element is added?
Does R7RS-large have keyword arguments (or will it)? I think that in common use cases of sets, the overwrite-vs-preserve policy is immaterial and any reasonable default would be OK. But there are rarer uses cases when one or the other behavior is crucial. A keyword argument seems like a clean way of accommodating the rare cases without cluttering up the common ones.