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

  Returns the strict relation corresponding to the binary relation 
  BinRel1.

    1> R1 = sofs:relation([{1,1},{1,2},{2,1},{2,2}]),
    R2 = sofs:strict_relation(R1),
    sofs:to_external(R2).
    [{1,2},{2,1}]
