The reason why this cannot be reproduced for Tr is that this predicate is always a predicate of a language (the metalanguage) different from the language of the sentences to which it applies (the object language). It is not possible to form a sentence of the language for which one defines Tr that says of itself that it is (not) Tr, since Tr is not a predicate of that language. On the other hand, it is certainly possible in some cases to form a sentence S of the metalanguage that says of itself that it is not Tr, but since S is a sentence of the metalanguage it is simply true and (presumably) not paradoxical, since there is no reason why a biconditional like (b) should hold for it; biconditionals of this kind hold only for sentences of the language for which Tr has been defined, not for sentences of the metalanguage.