
  -spec product(Set1, Set2) -> BinRel
                   when
                       BinRel :: binary_relation(),
                       Set1 :: a_set(),
                       Set2 :: a_set().

  Returns the Cartesian product of Set1 and Set2.

    1> S1 = sofs:set([1,2]),
    S2 = sofs:set([a,b]),
    R = sofs:product(S1, S2),
    sofs:to_external(R).
    [{1,a},{1,b},{2,a},{2,b}]

  product(S1, S2) is equivalent to product({S1, S2}).
