docs: private void generateSubTitles(final Metadata m, final Element e) {
          for (final SubTitle subTitle : m.getSubTitles()) {
              final Element subTitleElement = new Element("subTitle", NS);
              addNotNullAttribute(subTitleElement, "type", subTitle.getType());
              addNotNullAttribute(subTitleElement, "lang", subTitle.getLang());
              addNotNullAttribute(subTitleElement, "href", subTitle.getHref());
              if (subTitleElement.hasAttributes()) {
                  e.addContent(subTitleElement);
              }
          }
      }