theorem
  for g being Function of X,X holds F.:(id X, g)*f = F.:(f,g*f)
proof
  let g be Function of X,X;
  thus F.:(id X, g)*f = F.:(id X*f, g*f) by Th25
    .= F.:(f,g*f) by FUNCT_2:17;
end;
