new day, french energy, voodoo is kicking in
looks viable
voodoo starting to become voodoo. avoiding typer crash by not using alias type N = Node[B, L] in branch like. damn
step
step
before figuring out how to extend branch updates
getting more nasty again
actually... do we need LeafLike[A] at all?
enough to have abstract leaf
looks reasonable? we could override stuff to directly have Expr.Var, but using an extractor should be fine for now