Returns an arbitrary member or pair of a set, bag, or map. Specifically,
on a nonempty set, returns two values, an arbitrary member of the set and
true; on a nonempty bag, returns an arbitrary member, its multiplicity,
and true; on a nonempty map, returns an arbitrary domain member, its
associated value, and true. On an empty set, bag, or map, returns false for
all values. Please note that “arbitrary” does not mean “randomly selected”;
it simply means that the sole postcondition is that the returned value or pair
is a member of the collection.