private static void assertFieldName(OperatorNode<?> ast) {
        Preconditions.checkArgument(ast.getOperator() == ExpressionOperator.READ_FIELD ||
                        ast.getOperator() == ExpressionOperator.PROPREF,
                "Expected operator READ_FIELD or PRPPREF, got %s.", 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