docs: def __create_index(self, keys, index_options):
          """Internal create index helper.
  
          :Parameters:
            - `keys`: a list of tuples [(key, type), (key, type), ...]
            - `index_options`: a dict of index options.
          """
          index_doc = helpers._index_document(keys)
          index = {"key": index_doc}
          collation = validate_collation_or_none(
              index_options.pop('collation', None))
          index.update(index_options)
  
          with self._socket_for_writes() as sock_info:
              if collation is not None:
                  if sock_info.max_wire_version < 5:
                      raise ConfigurationError(
                          'Must be connected to MongoDB 3.4+ to use collations.')
                  else:
                      index['collation'] = collation
              cmd = SON([('createIndexes', self.name), ('indexes', [index])])
              try:
                  self._command(
                      sock_info, cmd, read_preference=ReadPreference.PRIMARY,
                      codec_options=_UNICODE_REPLACE_CODEC_OPTIONS,
                      write_concern=self.write_concern,
                      parse_write_concern_error=True)
              except OperationFailure as exc:
                  if exc.code in common.COMMAND_NOT_FOUND_CODES:
                      index["ns"] = self.__full_name
                      wcn = (self.write_concern if
                             self.write_concern.acknowledged else WriteConcern())
                      self.__database.system.indexes._insert(
                          sock_info, index, True, False, False, wcn)
                  else:
                      raise