
[;1m  inverse_image(BinRel, Set1)[0m

  Returns the inverse image of [;;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([c,d,e]),
    S2 = sofs:inverse_image(R, S1),
    sofs:to_external(S2).
    [2,3]
