How to use the multimethod.__init__.subtype.subcheck function in multimethod

def __setitem__(self, types: tuple, func: Callable):
        self.clean()
        types = signature(types)
        parents = types.parents = self.parents(types)
        for key in self:
            if types < key and (not parents or parents & key.parents):
                key.parents -= parents
                key.parents.add(types)
        if any(map(subtype.subcheck, types)):
            self.get_type = get_type  # switch to slower generic type checker
        super().__setitem__(types, func)
        self.__doc__ = self.docstring