public Parser assignClause() {
        if (assignClauseParser == null) {
           FutureParser future = scoped("assignClause");
           assignClauseParser = future;
           future.setParser(
               sequence(
                   token("ASSIGN"),
                   choice(
                       assignUsingClause(),
                       assignToClause()
                   )
               )
           );
        }

        return assignClauseParser;
    }