docs: private static int validate(byte[] decodabet, byte from) {
          byte b = decodabet[from & 0x7F];
          if (b < 0) {
              throw new IllegalArgumentException(String.format(
                      "Bad Base64%s character '%s'",
                      (decodabet == _URL_SAFE_DECODABET ? " url safe" : ""),
                      escape(from)));
          }
          return b;
      }