`BYWEEKNO` used with `FREQ=MONTHLY` is invalid

[axunonb]

As of Section 3.3.10 of RFC 5545 The BYWEEKNO rule part (which specifies ordinals for weeks of the year) MUST NOT be used when the frequency is anything other than YEARLY.

DTSTART:20260101T090000Z
RRULE:FREQ=MONTHLY;BYWEEKNO=15;UNTIL=20261231T090000Z

This returns 09 Apr 2026 11:00:00 instead of undefined. Is this on purpose?

Also

DTSTART:20260101T090000Z
RRULE:FREQ=MONTHLY;BYSETPOS=1

This returns the first day of each month, while this is also undefined:
The BYSETPOS rule part (which specifies the nth occurrence within a set) MUST only be used in conjunction with another BYxxx rule part.

[ggaabe]

We were intentionally lenient and had a test for this. Added a strict RFC 5545 enforcement option now:

#96

[axunonb]

Nice, thanks. We have a similar discussion re lenient vs. strict RFC5545 on ical.net. That's why I double-checked how you're dealing with it.