code: def get_rel_path_fragment(self, doc_id):
          
          with self._index_lock:
              r = self._doc_index[doc_id]
          fp = r[-1]
          return fp[(len(self.path) + 1):]
docs: For `doc_id` returns the path from the
          repo to the doc file. This is useful because
          (if you know the remote), it lets you construct the full path.