Binary Tree Traversal In LISP(Preorder,Inorder & PostOrder)

gistfile1.txt

(defparameter *tree2* '(40 (30 (25 () ()) (35 () ())) (60 (50 () ()) ())))
(defparameter *tree* '(3 (2 (1 () ()) ()) (5 () ())))

;validate if given parameter is a binary tree or not
(defun is-tree(tree)
    (cond 
         ((null tree) NIL)
         ((< (list-length tree) 3) NIL)
         (t)
    )
)

;return left subtree of a binary tree
(defun left-subtree(tree)
    (cond 
         ((null tree) NIL)
         ((not (listp tree)) NIL)
         (t (car (cdr tree)))
    )
)

(defun right-subtree(tree)
    (cond 
         ((null tree) NIL)
         ((not (listp tree)) NIL)
         (t (car (cdr (cdr tree))))
    )
)

;perform preorder traverse
(defun preorder(tree)
    (if 
        (not (is-tree tree)) NIL
        
           (cons (car tree)
               (append (preorder (left-subtree tree))
                     (preorder (right-subtree tree))
               )
           )
    )
)

;perform inorder traverse
(defun inorder(tree)
    (if 
        (not (is-tree tree)) NIL
            (append 
                 (inorder (left-subtree tree))
                 (list (car tree))
                 (inorder (right-subtree tree))
            ) 
    )
)

;perform postorder traverse
(defun postorder(tree)
    (if 
        (not (is-tree tree)) NIL
            (append 
                 (postorder (left-subtree tree))
                 (postorder (right-subtree tree))
                 (list (car tree))
            ) 
    )
)

;example execution
(print (postorder *tree2*))
(print (preorder *tree2*))
(print (inorder *tree2*))

how do you return values from the level on the list?