(define rl
  (lambda ()
    (load "second.ss")))

;;; are atom? and pair? YES!
(define leftmost
  (lambda (ls)
    (cond [(atom? ls) ls]
	  [else (leftmost (car ls))])))

;;; do not reverse the list
(define rightmost
  (lambda (ls)
    (cond [(atom? ls) ls]
	  [(null? (cdr ls)) (rightmost (car ls))]
	  [else (rightmost (cdr ls))])))

(define count-parens-all
  (lambda (ls)
    (cond [(null? ls) 2]
	  [(pair? (car ls)) (+ (count-parens-all (car ls))
			       (count-parens-all (cdr ls)))]
	  [(null? (car ls)) (+ 2 (count-parens-all (cdr ls)))]
	  [else (count-parens-all (cdr ls))])))
	   

;;; visit each element just once
;;; dos not work yet, does not work when item is not in car
(define remove-leftmost
  (lambda (e ls)
    (cond [(null? ls) '()]
	  [(equal? e (car ls)) (cdr ls)]
	  [(pair? (car ls)) (cons (remove-leftmost e (car ls))
				  (cdr ls))]
	  [else (cons (car ls) (remove-leftmost e (cdr ls)))])))


;;; remove-rightmost


;;; reverse-all p.107 (requires accu)
(define reverse
  (lambda (ls accu)
    (cond [(null? ls) accu]
	  [(pair? (car ls)) (reverse (cdr ls) (cons (reverse (car ls) '()) accu))]
	  [else (reverse (cdr ls) (cons (car ls) accu))])))

;;; flatten p.111
(define flatten
  (lambda (ls)
    (cond [(null? ls) '()]
	  [(pair? (car ls)) (append (flatten (car ls))
				    (flatten (cdr ls)))]
	  [else (cons (car ls) (flatten (cdr ls)))])))