(define sumtree
(lambda (x)
(if (eq (type-of x) type-i)
x
(if (eq x 'nil)
0
(let ((a (sumtree (car x)))
(b (sumtree (cdr x))))
(+ a b)
)))))
(check (= (sumtree (list (list 1 1 1 1 1 1 1 1 1 1) (list 1 2) (list 3 4))) 20))