; etude on a binary tree

(define xs '(1 (2 (4 (8 () ()) (9 () ()))
(5 (10 () ()) (11 () ()))) (3 (6 (12 () ())
(13 () ())) (7 (14 () ()) (15 () ())))))

(define (f xs)
  (if (null? xs) 0
    (- (car xs) (f (cadr xs)) (f (caddr xs)))))

(display (f xs)) (newline)