Thinking of lookup data.
   Say the root were included in every tip update. The root could
   reference the locations where its immediate child internodes are. The
   important ones would be the final updates that fill a subtree, as these
   have access to the location of every leaf.
   Before a subtree is filled, what to include seems a little hazy. The
   first leaf needs a reference to itself, and the left largest subtree,
   to pretend to be a new root.
   On the second subtree leaf, a new subroot is made to hold both of them.
   so it contains two indexing nodes: one to reference the last biggest
   subtree, and one to reference its pairing with its neighbor.
   Then the third subtree leaf actually only needs two again: a root that
   references the old root and the larger subroot, and then the larger
   subroot that references its sibling subtree and itself.
   In a graph, basically an index is needed for what ....
   Base root. Parenting subroot. It has a log2 in it somewhere i guess.
   The big question is how to build indexes based on last state, that
   develop the same minimal number of indexing entries. The idea comes
   from imagining balanced subtrees that never move and never change after
   being filled.