private OperatorNode<SortOperator> convertSortKey(Orderby_fieldContext node, Scope scope) {
        TerminalNode descDef = node.DESC();
        OperatorNode<ExpressionOperator> exprNode = convertExpr(node.expression(), scope);
        if (descDef != null ) {
            return OperatorNode.create(toLocation(scope, descDef), SortOperator.DESC, exprNode);
        } else {
            return OperatorNode.create(toLocation(scope, node), SortOperator.ASC, exprNode);
        }
    }
Translate the ANTLR grammar into the logical representation.
container-search/src/main/java/com/yahoo/search/yql/ProgramParser.java