docs: def index_in_tree(self, cmd_path):
          """
          Return the start index of which the element is not in cmd tree.
          :type cmd_path: list or tuple
          :return: None if cmd_path already indexed in tree.
          """
          current_tree = self.tree
          for key in cmd_path:
              if key in current_tree['children']:
                  current_tree = current_tree['children'][key]
              else:
                  return cmd_path.index(key)
          return None