An event with an RRULE can have extra VEVENT's which share their UID,
but add a RECCURENCE-ID which contains when the event was supposed to
take place. In place of that time it may supply it's own overriding
time. It may also override other fields.
Quick-case is a horrible macro. Before it was both hard to see what it
did, and how it did it. That still holds true, but at least the code for
it is ever so slightly more readable.