Related smells: Incomplete Conditional
The designers of grammar notations, as all DSL designers, try to make them fit 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. Single-element sequences, disjunctions and conjunctions like this are easy to detect and remove by algebraic rewriting.