[chombier]

If you're going to introduce a generic functor fixpoint then you'll probably want to define

     type Tree a = Fix (TreeF a)

so that `toTreeF` is just `unfix` (and is effectively free, as I mentioned above)

[solomonb]

Yeah you could do that but now you have thrown away the recursive `Tree` type and are always working in `TreeF`. This is totally fine but is a different design decision from where we started.