F# Monadically label n-ary tree

// F# Monadically label n-ary tree
let MMap f xs = 
            let rec MMap' (f, xs', out) = 
                state {
                        match xs' with
                        | h :: t -> let! h' = f(h)
                                    return! MMap'(f, t, List.append out [h'])
                        | [] -> return out
                      }
            MMap' (f, xs, [])

type 'a ntree = | NLeaf of 'a
                | NNode of 'a ntree list


let rec private MNNode (x) =
    match x with
    | NLeaf(c) -> state { let! x = GetState
                          do! SetState (x + 1)
                          return NLeaf(c,x) }
                         
    | NNode(xs) -> state { let! xs' = MMap MNNode xs
                           return NNode(xs')  } 
                    
let MNLabel x = Execute(MNNode(x))