Soulmates

If enough information is available about the evolution of the grammar (e.g., in a form of a versioned repository or a piece of documentation describing all changes), one can notice two nonterminals having a so-called co-change relationship when each revision that changes one, also changes the other. The smell occurs when this co-change relationship in the revision log does not correspond to explicit dependencies between modules and nonterminals.

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.