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

  Returns the inverse image of Set1 under the binary relation 
  BinRel.

    1> R = sofs:relation([{1,a},{2,b},{2,c},{3,d}]),
    S1 = sofs:set([c,d,e]),
    S2 = sofs:inverse_image(R, S1),
    sofs:to_external(S2).
    [2,3]
