let rec countA (a:int) = function
    | Leaf          -> 0
    | Node(tl,n,tr) -> countA a tl + countA a tr + (a+1)

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

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

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)))