
  -spec field(BinRel) -> Set
                 when BinRel :: binary_relation(), Set :: a_set().

  Returns the field of the binary relation BinRel.

    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]

  field(R) is equivalent to union(domain(R), range(R)).
