Add volume method.
New volumeIndex method.
Backup monoidic volume tree.
Revert to monoid free tree.
Remove unused code.
Remove unnecessary Root.
Remove unused Root class.
Add update method.
Simplify LeafN.
Simplify.
Simplify.
Simplify.
Simplify.
Simplify.
Simplify.
Simplify.
Simplify.
Simplify.
Simplify.
Simplify.