A checkin is ‘copy this list fragment from that list over the top of this fragment’.
If a bit is unchanged as a result of the checkin (ie, everything else), then conceptually it gets moved to the new list and replaced with a reference. Cool. Now, if the physical I’d is unchanged, this means that physical ids will always point to the latest occurrence of the item. The item needs to be usdataed to say “I belong to this list, now”. A node should be marked, then, with the place where it was first made. This way, we can say that a list contains Polychaeta as per AFD timestamp-to-timestamp.