code: private static int headingIndex(Element element) {
      String tagName = element.tagName();
      if (tagName.startsWith("h")) {
        try {
          return Integer.parseInt(tagName.substring(1));
        }
        catch (Throwable ex) {
          throw new IllegalArgumentException("Must be a header tag: " + tagName, ex);
        }
      }
      else {
        throw new IllegalArgumentException("Must be a header tag: " + tagName);
      }
    }
docs: Retrieves numeric index of a heading.
  @param element
  @return Index