Get the delegate FS for a given path.

        Arguments:
            path (str): A path.

        Returns:
            (FS, str): a tuple of ``(<fs>, <path>)`` for a mounted filesystem,
            or ``(None, None)`` if no filesystem is mounted on the
            given ``path``.

def _delegate(self, path):
        # type: (Text) -> Tuple[FS, Text]
        """Get the delegate FS for a given path.

        Arguments:
            path (str): A path.

        Returns:
            (FS, str): a tuple of ``(<fs>, <path>)`` for a mounted filesystem,
            or ``(None, None)`` if no filesystem is mounted on the
            given ``path``.

        """
        _path = forcedir(abspath(normpath(path)))
        is_mounted = _path.startswith

        for mount_path, fs in self.mounts:
            if is_mounted(mount_path):
                return fs, _path[len(mount_path) :].rstrip("/")

        return self.default_fs, path