sS weekday_after_ymdSlashes(int weekdayNr, S date) {
  if (weekdayNr < 1) null;
  int day1 = dayOfWeek_nr(parseYYYYslashMMslashDD(date));
  ret datePlusDays_slashes(date, mod(weekdayNr-day1-1, 7)+1);
}