docs: public List<VoltXMLElement> extractSubElements(String elementName, String attrName, String attrValue) {
          assert(elementName != null);
          assert((elementName != null && attrValue != null) || attrName == null);
          List<VoltXMLElement> elements = new ArrayList<>();
          extractSubElements(elementName, attrName, attrValue, elements);
          return elements;
      }