I remember there's an old mainstream project that does this, but I
   don't remember what it is off hand very well. Anyway it basically just
   used whatever the clearest approach was.
   First update is first root.
   Second update makes two leaves. The new root goes in the second update,
   with both leaves its children.
   Third update makes a third leaf and third root. The tree is binary and
   the third leaf has a sibling that is waiting for the 4th update.
   There is an internode between the third leaf and the third root that is
   the parent of the 4th root. With the original system, I believe this
   internode can be considered implicit, since everything is always a
   balanced binary tree. Hence, only the data and leaf index need be
   stored via the storage medium, in the extreme.
   Compare my approach. Ideally I would make the same simple balanced
   trees, but maybe with more internode information since leaves are not
   always at the end. We could set a norm for implicit internode
   information, if not provided, to optimize append-only cases further.
   Let's see if I can stabilize this in my mind a little ... in the
   previous way, a binary tree is built up from one side to the other,
   like a card tower. For updates that are two powers, a new root is made:
   like a new row of cards except in a hierarchical binary way.
   First one node.
   Then three.
   Then seven: two threes with a seventh as the root.
   That third root looks helpful for simple description. When the 4th node
   is added, it is basically saying there is a new root with 1 subtree and
   1 leaf as children: anything with comparable topology or what it. Then
   the 5th node becomes a sibling to the 4th. What data would be in a 5th
   update?