rem: iso_week_days (yday, wday) int yday; int wday;
add: iso_week_days (int yday, int wday)
context: iso_week_days (yday, wday)     int yday;     int wday;{  /* Add enough to the first operand of % to make it nonnegative.  */  int big_enough_multiple_of_7 = (-YDAY_MINIMUM / 7 + 2) * 7;  return (yday	  - (yday - wday + ISO_WEEK1_WDAY + big_enough_multiple_of_7) % 7	  + ISO_WEEK1_WDAY - ISO_WEEK_START_WDAY);}