@Override
 public Expression caseExpression_String(Expression_String object) {
   String value = object.getValue();
   if (value == null || value.isEmpty()) {
     return EngineExpressionUtil.createConstantExpression(
         "<empty-string>", "", String.class.getName());
   }
   return EngineExpressionUtil.createConstantExpression(value, value, String.class.getName());
 }
 @Override
 public Expression caseExpression_Double(Expression_Double object) {
   return EngineExpressionUtil.createConstantExpression(
       String.valueOf(object.getValue()),
       String.valueOf(object.getValue()),
       Double.class.getName());
 }
 @Override
 public Expression caseExpression_Boolean(Expression_Boolean object) {
   return EngineExpressionUtil.createConstantExpression(
       String.valueOf(object.isValue()),
       String.valueOf(object.isValue()),
       Boolean.class.getName());
 }
 @Override
 public Expression caseExpression_Integer(Expression_Integer object) {
   return EngineExpressionUtil.createConstantExpression(
       String.valueOf(object.getValue()), String.valueOf(object.getValue()), Long.class.getName());
 }