Adds another type X of typecheck annotations to the framework.
        predicate(annot) indicates whether annot has annotation type X;
        factory(annot) creates the appropriate typechecker instance.
        The checker type is normally added after the existing ones,
        but 'prepend' makes it come first.

def register(cls, predicate, factory, prepend=False):
        """
        Adds another type X of typecheck annotations to the framework.
        predicate(annot) indicates whether annot has annotation type X;
        factory(annot) creates the appropriate typechecker instance.
        The checker type is normally added after the existing ones,
        but 'prepend' makes it come first.
        """
        if prepend:
            cls._registered.insert(0, (predicate, factory))
        else:
            cls._registered.append((predicate, factory))