
  [;1m-spec is_equal(AnySet1, AnySet2) -> Bool[0m
  [;1m                  when[0m
  [;1m                      AnySet1 :: anyset(),[0m
  [;1m                      AnySet2 :: anyset(),[0m
  [;1m                      Bool :: boolean().[0m

  Returns [;;4mtrue[0m if [;;4mAnySet1[0m and [;;4mAnySet2[0m are equal, otherwise [;;4m[0m
  [;;4mfalse[0m. The following example shows that [;;4m==/2[0m is used when
  comparing sets for equality:

    1> S1 = sofs:set([1.0]),
    S2 = sofs:set([1]),
    sofs:is_equal(S1, S2).
    true
