Like :py:meth:`normalize.visitor.VisitorPattern.aggregate`, but
        returns .  This will normally only get called with a single type.

def itemtypes(cls, mapped_types, coll_type, visitor):
        """Like :py:meth:`normalize.visitor.VisitorPattern.aggregate`, but
        returns .  This will normally only get called with a single type.
        """
        rv = list(v for k, v in mapped_types)
        return rv[0] if len(rv) == 1 else rv