quiz.ss

(define rac
  (lambda (ls)
    (if (null? (cdr ls))
	(car ls)
	(rac (cdr ls)))))

(define rdc
  (lambda (ls)
    (if (null? (cdr ls))
	'()
	(cons (car ls) (rdc (cdr ls))))))

(define difference
  (lambda (los1 los2)
    (cond
      ((null? los1) '())
      ((memq (car los1) los2)
       (difference (cdr los1) los2))
      (else (cons (car los1) 
	      (difference (cdr los1) los2))))))

(define length*
  (lambda (nlos)
    (cond
      ((null? nlos) 0)
      ((symbol? (car nlos))
       (+ 1 (length* (cdr nlos))))
      (else
	(+ (length* (car nlos))
	  (length* (cdr nlos)))))))

(define membern?
  (lambda (sym n los)
    (cond
      ((zero? n) #t)
      ((null? los) #f)
      ((eq? sym (car los))
       (membern? sym (sub1 n) (cdr los)))
      (else (membern? sym n (cdr los))))))