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

  Returns the domain of the binary relation BinRel.

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