docs: public static VoltXMLElement mergeTwoElementsUsingOperator(String opName,
                                                                 String opElementId,
                                                                 VoltXMLElement first,
                                                                 VoltXMLElement second) {
          if (first == null || second == null) {
              return first == null ? second : first;
          }
          if (opName == null || opElementId == null) { return null; }
  
          VoltXMLElement retval = new VoltXMLElement("operation");
          retval.attributes.put( "id", opElementId );
          retval.attributes.put( "optype", opName );
          retval.children.add( first );
          retval.children.add( second );
          return retval;
      }