mhical rejecting "last <x>day in the month"

mhical rejects things like -1SU (last Sunday in the month), despite its
presence (at a cursory glance) in the ical standard:

  sbr/datetime.c:295:
    if (*cp == '+') { ++cp; } /* +n specific day; don't support '-' */
    else if (*cp == '-') { goto fail; }

.. so RRULEs like "FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU," fail.

Is this just a "not yet implemented", so possibly worth my trying to add?

Conrad