
  [;1m-spec image(BinRel, Set1) -> Set2[0m
  [;1m               when[0m
  [;1m                   BinRel :: binary_relation(),[0m
  [;1m                   Set1 :: a_set(),[0m
  [;1m                   Set2 :: a_set().[0m

  Returns the image of set [;;4mSet1[0m under the binary relation [;;4m[0m
  [;;4mBinRel[0m.

    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]
