code: public static List<String> parseLetterLinks(Document aDocument) {
      final List<String> results = new ArrayList<>();
  
      Elements links = aDocument.select(LETTER_URL_SELECTOR);
      links.forEach(element -> {
        if (element.hasAttr(ATTRIBUTE_HREF)) {
          String subpage = element.attr(ATTRIBUTE_HREF);
          results.add(OrfConstants.URL_BASE + subpage);
        }
      });
  
      return results;
    }
docs: determines the links to the letter pages
  
  @param aDocument the html document with letter links
  @return list with urls