Muster

leaves :: Tree a -> Int
leaves t = case t of
    Leaf ->  1
    Node { key = k, left = l, right = r } 
         -> leaves l + leaves r
inorder :: Tree a -> [a]
inorder t = case t of
    Leaf -> []
    Node { key = k, left = l, right = r } 
         -> inorder l ++ [ k ] ++ inorder r