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The lack of definition for nonterminals that are used within the grammar, is an obvious mistake that must be reported one way or another, and also possibly as a smell.

There are three main reasons for undefined nonterminals: (1) they were forgotten by the grammar engineer; (2) they are defined in a different module; (3) they are defined on a separate conceptual layer.

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.