
[;1m  converse(BinRel1)[0m

  Returns the converse of the binary relation [;;4mBinRel1[0m.

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