theorem Th37:
  for f,g being Function of X,Y holds f +* g = g
proof
  let f,g be Function of X,Y;
  per cases;
  suppose Y = {} implies X = {};
    then dom f = X & dom g = X by FUNCT_2:def 1;
    hence thesis by Th19;
  end;
  suppose Y = {} & X <> {};
    then f = {} & g = {};
    hence thesis;
  end;
end;
