theorem
  F.:(id X, id X)*f = F.:(f,f)
proof
  thus F.:(id X, id X)*f = F.:(id X*f, id X*f) by Th25
    .= F.:(id X*f, f) by FUNCT_2:17
    .= F.:(f,f) by FUNCT_2:17;
end;
