let rec count = function
 | Leaf          -> 0
 | Node(tl,n,tr) -> count tl + count tr + 1;;

let rec countA tree n =
 match tree with
 | Leaf -> n
 | Node(tl,_,tr) -> countA tl (countA tr (n+1));;

let rec countAC tree n c =
 match tree with
 | Leaf -> c n
 | Node(tl,_,tr) -> countAC tl (n+1) (fun vl -> countAC tr vl c);;

let rec countC t c =
 match t with
 | Leaf          -> c 0
 | Node(tl,n,tr) -> countC tl (fun vl -> countC tr (fun vr -> c(vl+vr+1)));;