private static void assertHasOperator(OperatorNode<?> ast, Class<? extends Operator> expectedOperatorClass) {
        Preconditions.checkArgument(expectedOperatorClass.isInstance(ast.getOperator()),
                                    "Expected operator class %s, got %s.",
                                    expectedOperatorClass.getName(), ast.getOperator().getClass().getName());
    }
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