
  [;1m-spec union(SetOfSets) -> Set[0m
  [;1m               when Set :: a_set(), SetOfSets :: set_of_sets().[0m

  Returns the union of the set of sets [;;4mSetOfSets[0m.

  [;1m-spec union(Set1, Set2) -> Set3[0m
  [;1m               when Set1 :: a_set(), Set2 :: a_set(), Set3 :: a_set().[0m

  Returns the union of [;;4mSet1[0m and [;;4mSet2[0m.
