
  [;1m-spec field(BinRel) -> Set[0m
  [;1m               when BinRel :: binary_relation(), Set :: a_set().[0m

  Returns the field of the binary relation [;;4mBinRel[0m.

    1> R = sofs:relation([{1,a},{1,b},{2,b},{2,c}]),
    S = sofs:field(R),
    sofs:to_external(S).
    [1,2,a,b,c]

  [;;4mfield(R)[0m is equivalent to [;;4munion(domain(R), range(R))[0m.
