docs: def keys_at(self, depth, counter=1):
          """
          Iterate keys at specified depth.
          """
          if depth < 1:
              yield ROOT
          else:
              if counter == depth:
                  for key in self.keys():
                      yield key
              else:
                  counter += 1
                  for dict_tree in self.values():
                      for key in dict_tree.keys_at(depth, counter):
                          yield key