How do I mark all iCal calendar events as read?

The original question (which I can't seem to enter an answer for) was on this thread: https://discussions.apple.com/thread

I found the answer on StackExchange (where knowledgeable folks actually answer questions):

1. Click on the [+] button next to [Calendars] at the top of the iCal app.
2. Once the list of notifications is in view, hold down the [Alt] key.
3. This will cause all of the [OK] buttons to change to [Ok All] and all the [Accept] buttons will change to [Accept All].
4. Clicking any [Ok All] will clear all invitations.

Reference: https://apple.stackexchange.com/questions/180513/quick-way-to-clear-calendar-notifications