docs: def prefix_dict_keys(d: Dict[str, Any], prefix: str) -> Dict[str, Any]:
      """
      Returns a dictionary that's a copy of as ``d`` but with ``prefix``
      prepended to its keys.
      """
      result = {}  # type: Dict[str, Any]
      for k, v in d.items():
          result[prefix + k] = v
      return result