;; Ćwiczenie 3.

(define (more e)
  (define (iter acc expr)
    (match expr
      [(op s l r) (match s
                    ['+ (iter (iter (inc acc) l) r)]
                    ['* (iter (iter (dec acc) l) r)])]
      [_ acc]))
  (let ([res (iter 0 e)])
    (cond [(= res 0) "="]
          [(> res 0) "+"]
          [else "*"])))

;; Ćwiczenie 4.

(struct text (title author chapter-sequence) #:transparent)
(struct chapter (title element-sequence)     #:transparent)
(struct subchapter (title element-sequence)  #:transparent)
(struct paragraph (string-sequence)          #:transparent)

(define (istext? book)
  (match book
    [(text t a c-s)     (and (string? t)
                             (string? a)
                             (andmap (lambda (x) (istext? x)) c-s))]
    [(chapter t e-s)    (and (string? t)
                             (andmap (lambda (x) (istext? x)) e-s))]
    [(subchapter t e-s) (and (string? t)
                             (andmap (lambda (x) (istext? x)) e-s))]
    [(paragraph s-s)    (andmap (lambda (x) (string? x)) s-s)]
    [_                  false]))