(define (atom? x) (not (pair? x)))
(define depth
  (lambda (x)
    (cond ((null? x) 0)
	  ((atom? x) 0)
	  (else (max (1+ (depth (car x)))
		     (depth (cdr x)))))))

(define sample (list 1 2 (list 3 4 (list 5 6))))
 (cdr sample)
(define sample2 (list ))

(depth sample)

(depth sample2)

(define nil '())

(depth (list ))

(depth (list nil))