
  -spec converse(BinRel1) -> BinRel2
                    when
                        BinRel1 :: binary_relation(),
                        BinRel2 :: binary_relation().

  Returns the converse of the binary relation BinRel1.

    1> R1 = sofs:relation([{1,a},{2,b},{3,a}]),
    R2 = sofs:converse(R1),
    sofs:to_external(R2).
    [{a,1},{a,3},{b,2}]
