docs: def _findUniqueMappingKeys(mapping):
      """Find mapping keys that only have one entry (value length of 1.
  
      :param mapping: dict, for each key contains a set of entries
      :returns: a set of unique mapping keys
      """
  
      uniqueMappingKeys = set()
      for key, entries in viewitems(mapping):
          if len(entries) == 1:
              uniqueMappingKeys.add(key)
      return uniqueMappingKeys