docs: private static String decodeBase900toBase10(int[] codewords, int count) throws FormatException {
      BigInteger result = BigInteger.ZERO;
      for (int i = 0; i < count; i++) {
        result = result.add(EXP900[count - i - 1].multiply(BigInteger.valueOf(codewords[i])));
      }
      String resultString = result.toString();
      if (resultString.charAt(0) != '1') {
        throw FormatException.getFormatInstance();
      }
      return resultString.substring(1);
    }