const time: PS.Parser<[number, number, number]> = PS.co(function* () {
  let h = yield digits(2, 2);
  yield PS.char(':');
  let m = yield digits(2, 2);
  // the seconds fields is optional
  let s = yield PS.option(0, PS.attempt(PS.char(':').chain(() => digits(2, 2))));

  if (h > 23 || m > 59 || s > 59) {
    return PS.fail('hour, minute and seconds must be in valid range');
  }
  return PS.pure([h, m, s]);
});

const decOctet: PS.Parser<string> = PS.co(function* () {
  const a1: string = yield countMinMax(1, 3, hexDigitChar).map(joinStr);
  let s = parseFloat(a1);
  if (isNaN(s) || s > 255) return PS.fail('Decimal octet value too large');
  return PS.pure(a1);
});

 return PS.attempt(p).chain(x => PS.fail('unexpected')).alt(PS.pure(void 0));