(define-struct tiger (name length sells))
;; a Tiger is a (make-tiger String Natural Product)
;;  inerp:  a tiger where
;;     name is the tiger's name
;;     length is the length of the tiger (in feet)
;;     sells is a Product that the tiger sells

(define-struct product (item company))
;; a Product is a (make-product String String)
;; interp:  a product where
;;    item is the name of the item
;;    company is the name of the company that manufactures the item

;;; fcn-for-product:  Product ->
;;;
;(define (fcn-for-product a-prod)
;  (... (product-item a-prod)
;       (product-company a-prod)))
;  

;; examples
(define TONY (make-tiger "Tony" 3 (make-product "Frosted Flakes" "Kellogg's")))

;; template

;;; fcn-for-tiger:  Tiger ->
;;;
;(define (fcn-for-tiger a-tiger)
;  (... (tiger-name a-tiger)
;       (tiger-length a-tiger)
;       (fcn-for-prod (tiger-sells a-tiger))))



;; sells-gas?:  Tiger -> Boolean
;; consumes a tiger and produces true if the item the tiger sells is gas
(define (sells-gas? a-tiger)   
       (is-it-gas? (tiger-sells a-tiger)))

;; test cases
(check-expect (sells-gas? (make-tiger "Tony" 3 (make-product "Frosted Flakes" "Kellogg's"))) false)
(check-expect (sells-gas? (make-tiger "Joe"  4 (make-product "gas" "Exxon"))) true)

;; is-it-gas?:  Product -> Boolean
;; returns true if the item is gas
(define (is-it-gas? a-prod)
  (string=? (product-item a-prod) "gas"))
       
;; test
(check-expect (is-it-gas? (make-product "cereal" "Kellogg's")) false)
(check-expect (is-it-gas?  (make-product "gas" "Exxon")) true)