A Key to sort the rules by weight for building.

        The key leads to ordering:

         - By routes with defaults first, as these must be evaulated
           for building before ones without.
         - Then the more complex routes (most converted parts).

def build_key(self) -> Tuple[bool, int]:
        """A Key to sort the rules by weight for building.

        The key leads to ordering:

         - By routes with defaults first, as these must be evaulated
           for building before ones without.
         - Then the more complex routes (most converted parts).
        """
        if self.map is None:
            raise RuntimeError(f"{self!r} is not bound to a Map")
        return (not bool(self.defaults), -sum(1 for weight in self._weights if weight.converter))