don't need synth.Sys
step
compiles but tree table view impl not finished
alright, we'll try to eliminate the type parameters for treelike update now
aha, this seems to work. step 1
step 2. left: Node[T#Branch, T#Leaf] ought to be Node[T]
ok, that should be it for [S, T] pairs. Node/IsLeaf/IsBranch shouldn't be touched because makes apply methods require type params
pick up tt view again
...