data Tree a = Empty            | Node (Tree a) a (Tree a) testTree : Tree StringtestTree = Node (Node (Node Empty "Jim" Empty) "Fred"                      (Node Empty "Sheila" Empty)) "Alice"                (Node Empty "Bob" (Node Empty "Eve" Empty)) flatten : Tree a -> List aflatten Empty = []flatten (Node left val right) = flatten left ++ val :: flatten right data State : (stateType : Type) -> Type -> Type where     Get : State stateType stateType     Put : stateType -> State stateType ()      Pure : ty -> State stateType ty     Bind : State stateType a -> (a -> State stateType b) ->             State stateType b {-(>>=) : State stateType a -> (a -> State stateType b) ->        State stateType b(>>=) = Bind-} mutual  Functor (State stateType) where      map func x = do val <- x                      Pure (func val)   Applicative (State stateType) where      pure x = Pure x      (<*>) f a = do func <- f                     arg <- a                     pure (func arg)   Monad (State stateType) where      (>>=) = Bind runState : State stateType a -> (st : stateType) -> (a, stateType)runState Get st = (st, st)runState (Put newState) st = ((), newState) runState (Pure x) st = (x, st)runState (Bind cmd prog) st = let (val, nextState) = runState cmd st in                                  runState (prog val) nextState treeLabelWith : Tree a -> State (Stream labelType) (Tree (labelType, a))treeLabelWith Empty = Pure EmptytreeLabelWith (Node left val right)     = do left_labelled <- treeLabelWith left          (this :: rest) <- Get          Put rest          right_labelled <- treeLabelWith right          Pure (Node left_labelled (this, val) right_labelled) treeLabel : Tree a -> Tree (Integer, a)treeLabel tree = fst (runState (treeLabelWith tree) [1..])