[morelisp]

A few times I have wished for a way to grow maps, but generally to shrink maps I don't think there's any big advantage to a built-in rather than your own function making a new one. (It would only cover the case where you know you want to shrink but don't know if you need to shrink because you don't know how many elements you removed or overestimated by, which I think is pretty unusual.)