We would need a way to transform a Tree of Trees into a single Tree. The inner Trees would be at leaf nodes of the outer Tree, so they could be attached in plac

We would need a way to transform a Tree of Trees into a single Tree. The inner Trees would be at leaf nodes of the outer Tree, so they could be attached in place. Although I recall monads need to satisfy certain laws, and I don't recall if Trees satisfy them or not. Just because something is a data structure does not necessarily imply it is a monad.