
  -spec from_sets(TupleOfSets) -> Ordset
                     when
                         Ordset :: ordset(),
                         TupleOfSets :: tuple_of(anyset()).

  -spec from_sets(ListOfSets) -> Set
                     when Set :: a_set(), ListOfSets :: [anyset()].

  Returns the unordered set containing the sets of list 
  ListOfSets.

    1> S1 = sofs:relation([{a,1},{b,2}]),
    S2 = sofs:relation([{x,3},{y,4}]),
    S = sofs:from_sets([S1,S2]),
    sofs:to_external(S).
    [[{a,1},{b,2}],[{x,3},{y,4}]]
