code: def unexpected(parser, at_token=None):
      # type: (Parser, Optional[Any]) -> GraphQLSyntaxError
      
      token = at_token or parser.token
      return GraphQLSyntaxError(
          parser.source, token.start, u"Unexpected {}".format(get_token_desc(token))
      )
docs: Helper function for creating an error when an unexpected lexed token
      is encountered.