docs: def del_by_idx(tree, idxs):
      """
      Delete a key entry based on numerical indexes into subtree lists.
      """
      if len(idxs) == 0:
          tree['item'] = None
          tree['subtrees'] = []
      else:
          hidx, tidxs = idxs[0], idxs[1:]
          del_by_idx(tree['subtrees'][hidx][1], tidxs)
          if len(tree['subtrees'][hidx][1]['subtrees']) == 0:
              del tree['subtrees'][hidx]