(define (insert tree value)
(cond ((eq? #t (contains? tree value))
       tree)
      (else
    (cond ((eq? (tree-leaf? tree) #t)  (make-tree value '() '()))
          (else
    (cond ((< value (tree-value tree)) (insert (tree-left tree) value))
          ((> value (tree-value tree)) (insert (tree-right tree) value))
          )
    )
          )
    )))