A follow-up to #368.

The current draft of the registry omits any way to define rules for validating runtime types, e.g. in message arguments. We're using regexes for literals (see #407), but there's no way for the :number function to specify that in case its argument is a variable ($count), the runtime value of that variable should meet some requirements (e.g. its type be in a union of some known types, or an instance of another known type).

It's not clear how to encode such requirements in an implementation-agnostic and programming-language-agnostic manner.