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

  Returns a subset S of the weak relation W corresponding to the
  binary relation BinRel1. Let F be the field of BinRel1. The
  subset S is defined so that x S y if x W y for some x in F and for
  some y in F.

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