
  -spec inverse(Function1) -> Function2
                   when
                       Function1 :: a_function(),
                       Function2 :: a_function().

  Returns the inverse of function Function1.

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