Singleton

The designers of grammar notations, as all DSL designers, try to make them t the domain, but never achieve absolute perfection. In particular, multiary symbols with arity of 2 and up, are commonly expressed in such a way that allows their use on an empty or trivial list of arguments.

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.