
[;1m  inverse(Function1)[0m

  Returns the inverse of function [;;4mFunction1[0m.

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