org-mac-iCal.el: work with calendar "groups"

org-mac-iCal.el: work with calendar "groups"

Doug Hellmann writes:

> Some calendar systems (Google, Zimbra) handle subscriptions to
> multiple calendars (or to an account) by grouping them under a single
> caldav directory in the calendar tree.  org-mac-iCal assumes there is
> only one ics file created per caldav directory, so while it *creates*
> all of the needed merged ics files, it only copies one of them to
> ~/Library/Calendar before importing the contents into the diary.