nsIMicrosummarySet in Firefox 2.0

 Add a microsummary observer to this microsummary set.  Adding an observer
 to a set is the equivalent of adding it to each constituent microsummary.

 @param observer
        the microsummary observer to add

 Retrieve a enumerator of microsummaries in the set.

 @returns an enumerator of nsIMicrosummary objects

 Remove a microsummary observer from this microsummary.

 @param observer
        the microsummary observer to remove