docs: private static boolean allHexDigits(String str, int start, int end)
      {
          boolean rtn = true;
          for (int i = start; i < end; ++i)
          {
              if ("0123456789abcdefABCDEF".indexOf(str.charAt(i)) == -1)
              {
                  rtn = false;
                  break;
              }
          }
          return rtn;
      }