docs: def _ensure_append(self, new_items, append_to, index=0):
          """Ensure an item is appended to a list or create a new empty list
  
          :param new_items: the item(s) to append
          :type new_items: list(obj)
          :param append_to: the list on which to append the items
          :type append_to: list()
          :param index: index of the list on which to append the items
          :type index: int
          """
          append_to = append_to or []
          append_to.insert(index, new_items)
          return append_to