docs: def _dict_native_ok(d):
      """
      This checks if a dictionary can be saved natively as HDF5 groups.
  
      If it can't, it will be pickled.
      """
      if len(d) >= 256:
          return False
  
      # All keys must be strings
      for k in d:
          if not isinstance(k, six.string_types):
              return False
  
      return True