Related smells: Unnecessary Hierarchy, Speculative Generality, Speculative Hierarchy
Formal language theory defines a language extensionally, as a set of all possible programs written in it. A grammar is an intensional definition, which is nicer because it is a finite specification of an infinitely large entity, but it also makes it harder to see some relations between that and the instances of the language. In particular, the actual codebase of the software language, if available and comprehensive enough, can serve as a good approximation of the language features used by programmers. If a grammar contains a feature that is never exercised by any program in the actual codebase, it is a Mythic feature that, for instance, does not have to be supported for a software analysis or migration tool to be useful and applicable.