data Bintree a = Empty | Fork a (Bintree a) (Bintree a) deriving Show

foldBtree :: val -> (a -> val -> val -> val) -> Bintree a -> val
foldBtree val _ Empty = val
foldBtree val f (Fork a left right) = f a (foldBtree val f left) (foldBtree val f right)

or_ :: Tree Bool -> Bool
or_  = foldTree id (||) False (||)

preorderB :: Bintree a -> [a]
preorderB = foldBtree [] (a : [] ++) -- here is the problem, please help for this function and i must use foldBtree to get this. THanks