private static void addStringItems(OperatorNode<ExpressionOperator> ast, WeightedSetItem out) {
        List<String> keys = ast.getArgument(0);
        List<OperatorNode<ExpressionOperator>> values = ast.getArgument(1);
        for (int i = 0; i < keys.size(); ++i) {
            OperatorNode<ExpressionOperator> tokenWeight = values.get(i);
            assertHasOperator(tokenWeight, ExpressionOperator.LITERAL);
            out.addToken(keys.get(i), tokenWeight.getArgument(0, Integer.class));
        }
    }
The sorting specified in the YQL+ query last parsed.

@return a Sorting instance or null
container-search/src/main/java/com/yahoo/search/yql/YqlParser.java