;; These functions are basically the same as on the slides

(define (len l)
  (if (null? l)
      0
      (+ 1 (len (cdr l)))))

(define (len-tr l accum)
  (if (null? l)
      accum
      (len-tr (cdr l) (+ 1 accum))))
(define (len2 l)
  (len-tr l 0))

(define (reverse1 l)
  (if (null? l)
      ()
      (append (reverse1 (cdr l)) (list (car l)))))

(define (reverse-tr l accum)
  (if (null? l)
      accum
      (reverse-tr (cdr l) (cons (car l) accum))))
(define (reverse2 l)
  (reverse-tr l ()))