+I'd work with a tree monadized with a `Continuation_monad`, that is, with:
+
+ #use "path/to/tree_monadize.ml";;
+
+You could in theory also use the richer "monads.ml" library:
+
+ #use "path/to/monads.ml";;
+ module TC = Tree_monad.T(Continuation_monad);;
+ let tree_monadize = TC.distribute;;
+
+But I found it easier to work out the solution using the simpler "tree\_monadize.ml" library. The "monadize.ml" library enforces encapsulation in a way that's nice and sanitized, but makes experimentation more cumbersome.
+
+
+It makes debugging easier if we work with a tree whose starting leaf
+elements are differently typed than the tree we aim to finish with. So:
+
+ let tree = Node(Leaf '1', Node(Leaf '2', Node(Leaf '3', Leaf '1')));;
+
+Now, we already know how to count the leaves using a Continuation monad
+in tree shape:
+
+ let v0 = TreeCont.monadize (fun a k -> 1 + k a) tree (fun t -> 0);;
+
+There are two interesting functions here: we'll call them the "distributed" function `fun a k -> 1 + k a` and the "seed" function `fun t -> 0`.
+
+Our first step is to extend `v0` so that we count how many leaves there are
+of each value, rather than how many leaves in total:
+
+ let update_env e x = fun y -> (if x = y then 1 else 0) + e y;;
+
+ let v1 = TreeCont.monadize (fun a k ->
+ fun e0 ->
+ let ecur = k a e0
+ in update_env ecur a
+ ) tree (fun t e -> e) (fun a -> 0);;
+
+ (* now
+ v1 '0' ~~> 0
+ v1 '1' ~~> 2
+ v1 '2' ~~> 1
+ *)
+
+How does this work? Our distributed function `fun a k -> ...` takes a leaf element `a` and a continuation `k`, which maps leaf elements and environments `e0` to new environments `ecur`. It gives back a function from `e0` to an updated version of `ecur`.
+
+Our seed function here is the initial continuation. Instead of taking a leaf element and an env, it takes a tree of such elements and an env. In general, wherever the distributed function takes `'a`s, the seed function takes `'a tree`s.
+
+Here our seed function just ignores the tree and returns the env unchanged.
+
+What that gives us is a function from environments to environments. We give it
+an initial environment `fun a -> 0` to get started. We get back an
+environment `v1` that maps each leaf element to how many times that leaf
+element appeared in the tree we started with.
+
+OK? You might want to stop reading here, and see how much further you
+can get on your own.
+
+But there are [more hints](/hints/assignment_10_hint_3) if you need them.