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

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

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