(define-datatype tree tree?
                 (tree-null)
                 (tree-node
                  (datum number?)
                  (left tree?)
                  (right tree?)))

(define add-tree
  (lambda (n t)
    (if (tree? t)
        (cases tree t
          (tree-null () (tree-node n t t))
          (tree-node (datum left right)
                     (if (< n datum)
                         (tree-node datum
                                    (add-tree n left)
                                    right)
                         (tree-node datum
                                    left
                                    (add-tree n right))))
          )
        'error-not-tree)
        )
  )

(define search-tree
  (lambda (n t)
    (if (tree? t)
        (cases tree t
          (tree-null () #f)
          (tree-node (datum left right)
                     (cond
                       ((eq? n datum) #t)
                       ((< n datum) (search-tree n left))
                       (else (search-tree n right))))
          )
        'error-not-tree)
    )
  )