Bug tracker is an example of something that extends both
cr_items and cr_revisions, although only having looked at the sql it looks like it does not in fact extend cr_items (even though th bt_bugs table pk references cr_items) since
it does:
select acs_object_type__create_type (
'bt_bug',
'Bug',
'Bugs',
'acs_object',
'bt_bugs',
'bug_id',
null,
'f',
null,
'bt_bug__name'
);
I guess Lars could comment on whether that is simply a bug
or is some sort of shortcoming with subclassing cr_item.