 One more step and we are home. As we prove the lemma, we should gather up any facts that can be stated in \(L\), are true in every domain, and are needed for proving the lemma. For example we shall almost certainly need the sentence saying that \(\subseteq\) is transitive. Write \(T\) for the set of all these sentences. (In Tarski’s presentation \(T\) vanishes, since he is using higher order logic and the required statements about classes become theorems of logic.) Thus we reach, for example: