def _min_depth(self):
        """Finds minimum path length from the root.

        Notes
        -----
          Internal method. Do not call directly.

        Returns
        -------
          int
        Minimum path length from the root.

        """
        if "min_depth" in self.__dict__:
            return self.__dict__["min_depth"]

        min_depth = 0
        hypernyms = self.hypernyms()
        if hypernyms:
            min_depth = 1 + min(h._min_depth() for h in hypernyms)
        self.__dict__["min_depth"] = min_depth

        return min_depth