Mythic

If a grammar contains a feature that is never exercised by any program in the actual codebase, it is a Mythic feature that, does not have to be supported for an analysis or migration tool to be useful and applicable.

Reference

Mats Stijlaart and Vadim Zaytsev. 2017. Towards a taxonomy of grammar smells. In Proceedings of the 10th ACM SIGPLAN International Conference on Software Language Engineering (SLE 2017). ACM, New York, NY, USA, 43-54.