In 7e48886fd3, `Node::Pruned` was
introduced in order to fix problems with `LocatedTree::max_position`
resulting from reinsertion of subtree information as children of
a previously pruned subtree represented by a `Node::Leaf` at its
root. This fix introduced a large amount of complexity that is
better resolved by fixing the `max_position` function in a different
way and generally minimizing its usage.
In circumstances where insertion into a subtree results in pruning, and
then a subsequent insertion within the same range contains leaves that
must be retained, it is necessary to be able to distinguish the maximum
position among notes that have been observed but later pruned. This
fixes a bug wherein an insertion into an already-pruned tree could cause
the maximum position reported for the subtree to regress.
Jack Grigg
Kris Nuttycombe