code: public static <T extends AbstractJaxb> List<T> getDescendants(T target,
              List<T> resultList, Class<T> tagType) {
          return execute(target, resultList, tagType, null);
      }
docs: タグ指定で子孫要素を返す
  
  @param <T>
  tag class type. (i.e. Div.class, Span.class...)
  @param target
  objects for scan
  @param resultList
  usually, pass new ArrayList
  @param tagType
  tag class
  @return