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

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

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