
  [;1m-spec weak_relation(BinRel1) -> BinRel2[0m
  [;1m                       when[0m
  [;1m                           BinRel1 :: binary_relation(),[0m
  [;1m                           BinRel2 :: binary_relation().[0m

  Returns a subset S of the weak relation W corresponding to the
  binary relation [;;4mBinRel1[0m. Let F be the field of [;;4mBinRel1[0m. 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}]
