docs: def items_at(self, depth):
          """
          Iterate items at specified depth.
          """
          if depth < 1:
              yield ROOT, self
          elif depth == 1:
              for key, value in self.items():
                  yield key, value
          else:
              for dict_tree in self.values():
                  for key, value in dict_tree.items_at(depth - 1):
                      yield key, value