A BadListProduction is a Production subclass for a noun list written in a slot intended for a single noun only. This isn't really a grammatical error, so the language grammar will probably want to include a rule for this. However, it *is* a semantic error; rules that are written for single objects are written that way because we can't make sense of a list there. For example, PUT BOOK IN BOX AND BAG would be nonsensical to us, because we can't put something in two places at once. This class can be used for a grammar rule that parses a list where a single noun is required; we'll flag it with an explanatory error message for the user.

class BadListProduction :   Production