reserve P,Q,X,Y,Z for set, p,x,x9,x1,x2,y,z for object;

theorem
  for f being Function holds f is Function of dom f, rng f
proof
  let f be Function;
  reconsider R = f as Relation of dom f, rng f by RELSET_1:4;
  rng R <> {} or rng R = {};
  hence thesis by Def1;
end;
