(define (ex1 v1 v2)
  (let ([f (let ([l '()])
             (lambda (x)
               (set! l (cons x l))
               l))])
    (begin
      (f v1)
      (f v2))))

(define (ex2 v1 v2)
  (let ([mk (let ([l '()])
              (lambda ()
                (lambda (x)
                  (set! l (append l x))
                  l)))])
    (let ([f (mk)]
          [g (mk)])
      (begin
        (f (list v1))
        (g (list v2))
        (g '())))))

(define (ex3 v1 v2)
  (let ([mk (lambda ()
              (let ([l '()])
                (lambda (x)
                  (set! l (append l x))
                  l)))])
    (let ([f (mk)]
          [g (mk)])
      (begin
        (g (list v1))
        (f (list v2))
        (g '())))))

(define (ex4 v1 v2 v3 v4)
  (let ([mk (let ([l '()]
                  [combine (lambda (x y) (append y x))])
              (lambda ()
                (let ([l2 '()]
                      [combine2 combine])
                  (set! combine append)
                  (lambda (x y)
                    (set! l (append l y))
                    (set! l2 (append l2 x))
                    (combine2 l l2)))))])
    (let ([f (mk)]
          [g (mk)])
      (begin
        (f (list v1) (list v2))
        (g (list v3) (list v4))
        (list (f '() '()) (g '() '()))))))