(set *tree* [1 [2 [4 [7]]
                  [5]]
               [3 [6 [8]
                     [9]]]])

(define inorder
  []      -> []
  [V]     -> [V]
  [V L]   -> (append (inorder L)
                     [V])
  [V L R] -> (append (inorder L)
                     [V]
                     (inorder R)))

(define postorder
  []      -> []
  [V]     -> [V]
  [V L]   -> (append (postorder L)
                     [V])
  [V L R] -> (append (postorder L)
                     (postorder R)
                     [V]))

(define preorder
  []      -> []
  [V]     -> [V]
  [V L]   -> (append [V]
                     (preorder L))
  [V L R] -> (append [V]
                     (preorder L)
                     (preorder R)))

(define levelorder-0
  []             -> []
  [[]       | Q] -> (levelorder-0 Q)
  [[V | LR] | Q] -> [V | (levelorder-0 (append Q LR))])

(define levelorder
  Node -> (levelorder-0 [Node]))

(preorder (value *tree*))
(postorder (value *tree*))
(inorder (value *tree*))
(levelorder (value *tree*))