Search trees by `sub_path` using ``Tree.path.startswith(sub_path)``
        comparison.

        Args:
            sub_path (str): Part of the :attr:`.Tree.path` property of
                :class:`.Tree`.

        Returns:
            set: Set of matching :class:`Tree` instances.

def trees_by_subpath(self, sub_path):
        """
        Search trees by `sub_path` using ``Tree.path.startswith(sub_path)``
        comparison.

        Args:
            sub_path (str): Part of the :attr:`.Tree.path` property of
                :class:`.Tree`.

        Returns:
            set: Set of matching :class:`Tree` instances.
        """
        matches = (
            self.path_db[tree_path].keys()
            for tree_path in self.path_db.iterkeys()
            if tree_path.startswith(sub_path)
        )

        return set(sum(matches, []))