private static void assertHasOperator(OperatorNode<?> ast, Operator expectedOperator) {
        Preconditions.checkArgument(ast.getOperator() == expectedOperator,
                                    "Expected operator %s, got %s.",
                                    expectedOperator, ast.getOperator());
    }
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