code: def _parse_canonical_maxkey(doc):
      
      if doc['$maxKey'] is not 1:
          raise TypeError('$maxKey value must be 1: %s', (doc,))
      if len(doc) != 1:
          raise TypeError('Bad $minKey, extra field(s): %s' % (doc,))
      return MaxKey()
docs: Decode a JSON MaxKey to bson.max_key.MaxKey.