code: public static String makeLuhnVerifiable (String key)
      {
          String prekey = key + '0';
          int rem = getLuhnRemainder(prekey);
  
          return (rem == 0 ? prekey : key + (10 - rem));
      }
docs: Takes a random string of digits and adds the appropriate check
  digit in order to make the whole string verifyable using {@link
  #verifyLuhn}.