;; 5.1
(define remove-all
  (lambda (l)
    (cond
     ((null? l) '())
     ((pair? (car l))
      (cons (remove-all (car l)) (remove-all (cdr l))))
     (else
      (remove-all (cdr l)))) ))

;; 5.2
; append is predefined, but here it is for reference.
(define append-lists
  (lambda (l1 l2)
    (cond
     ((null? l1) l2)
     (cons (car l1) (append-lists (cdr l1) l2)) )))

; snoc: put an item on the end of a list.
; used as a helper for reverse-list.
(define snoc
  (lambda (l x)
    (cond
     ((null? l) (list x))
     (else (cons (car l) (snoc (cdr l) x))))))

(define reverse-list
  (lambda (l)
    (cond
     ((null? l) '())
     ((pair? (car l))
      (snoc (reverse-list (cdr l)) (reverse-list (car l))))
     (else
      (snoc (reverse-list (cdr l)) (car l))))))

;; 5.3
;; make-single written with an accumulator to hold the 
;; previous value for comparison. (null is used for the empty value)
(define make-single-acc
  (lambda (l prev)
    (cond
     ((null? l) '())
     ((pair? (car l))
      (cons (make-single-acc (car l) '()) 
            (make-single-acc (cdr l) '())))
     (else
      (if (null? prev)
          (cons (car l) (make-single-acc (cdr l) (car l)))
          (if (eq? (car l) prev)
              (make-single-acc (cdr l) prev)
              (cons (car l) (make-single-acc (cdr l) (car l)))))))))

(define make-single
  (lambda (l)
    (make-single-acc l '())))