; 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)
OyBldHVkZSBvbiBhIGJpbmFyeSB0cmVlCgooZGVmaW5lIHhzICcoMSAoMiAoNCAoOCAoKSAoKSkgKDkgKCkgKCkpKQooNSAoMTAgKCkgKCkpICgxMSAoKSAoKSkpKSAoMyAoNiAoMTIgKCkgKCkpCigxMyAoKSAoKSkpICg3ICgxNCAoKSAoKSkgKDE1ICgpICgpKSkpKSkKCihkZWZpbmUgKGYgeHMpCiAgKGlmIChudWxsPyB4cykgMAogICAgKC0gKGNhciB4cykgKGYgKGNhZHIgeHMpKSAoZiAoY2FkZHIgeHMpKSkpKQoKKGRpc3BsYXkgKGYgeHMpKSAobmV3bGluZSk=