docs: private void generateThumbails(final Metadata m, final Element e) {
          for (final Thumbnail thumb : m.getThumbnail()) {
              final Element t = new Element("thumbnail", NS);
              addNotNullAttribute(t, "url", thumb.getUrl());
              addNotNullAttribute(t, "width", thumb.getWidth());
              addNotNullAttribute(t, "height", thumb.getHeight());
              addNotNullAttribute(t, "time", thumb.getTime());
              e.addContent(t);
          }
      }