danking/racket — comment on commit 6fe5ba36d0987ab9ff11c5b9af0888a6accf1077
File: collects/racket/set.rkt line 153
By offby1:

From the name, I'd guess that hash-keys returns _all_ the set's elements. You then throw all but the first away. This sounds like it might be slow. Perhaps there is a way to retrieve just a single hash key?