On the complement of one complexity class in another

We investigate the diagonalization theorems in [7] (Theorem 23), [3] (Theorem 6) and [10] (main Theorem) and show that they can be strengthened so as to be applicable to most complexity classes, not just to those closed under polynomial-time reducibility. Thus the applications in [7], [3] and [10] (e.g. P ≠ P ⇒ NP\P is not recursively presentable) are not peculiar to P, NP, PSPACE etc.; rather, they are examples of properties common to almost all "reasonable" complexity classes.