
theorem
  for f being Function, g being one-to-one Function
  holds (f +* g)*(g") = id rng g
proof
  let f be Function, g be one-to-one Function;
  rng(g") = dom g by FUNCT_1:33;
  hence (f +* g)*(g") = g*g" by Th8
    .= id rng g by FUNCT_1:39;
end;
