ADD does not INVALIDATE current PATTERN

An ADD does not INVALIDATE any RECURRENCE-ID in current set.  Thus it does 
not change the pattern of existing set.
If an ADD INVALIDATED the existing set, than you would be forced to send a 
new RRULE/RDATES/EXRULE/EXDATES with ADD.
In that case the ADD would be pointless, just include the new DATE in the 
new rules.

An ADD is equivalent to sending the invitee an REQUEST with a 
RECURRENCE-ID that he does not currently have.
The invitee WOULD take CHAIRS word for RECURRENCE-ID and add it to his 
calendar, storing off RECURRENCE-ID that CHAIR supplied.

The CHAIR does not really care either.  He is in control and just adds it 
to his set.