[racket] Take an element from a set

This looks like a reasonable way to get an element, but thinking
ahead, you will probably want something both like "first" (which you
have) and a corresponding "rest".

The problem is that which element you get from the "first" operation
is nondetermistic in a set.  Therefore, you have to either

- pass the returned first element to the rest operation, or

- accept that
  (set-union (set (set-item S)) (set-rest S))
  is not the same as S

A better design would be to use a different signature:

  get-one/rest : (forall (A) (Set A) -> (values A (Set A)))

This performs the split "atomically", so it can ensure that the
combination of "one" and "rest" is the same as the original set.

Shriram