The procedure `set-get-one/rest' returns a random element from the set and a new
set with that element removed. It is not guaranteed to return the same element
if it is called twice on the original set.

The procedure `set-get-one' only returns a random element from the set. This can
be used if the programmer simply needs some element from the set and is not
iterating over the set.

This was discussed here:
http://lists.racket-lang.org/users/archive/2011-September/047919.html

… performance)