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