;; copy and past this code into your solution file

;; -----------------------------------------------------------------------------
;; the library of xexpr functions 

;; An Xexpr.v2 is one of: 
;;  -- (cons Symbol [Listof SX.v2])
;;  -- (cons Symbol (cons [Listof Attribute] [Listof SX.v2]))
;; An Attribute has the shape: 
;;  (cons Symbol (cons String empty))
;; An SX.v2 is one of: 
;;  -- String 
;;  -- Xexpr.v2 

;; Xexpr -> Symbol 
;; retrieve the name of the element 
(define (xexpr-name xe) 
  (first xe))

;; Xexpr -> [Listof Attribute]
;; retrieve the list of attributes of @racket[xe]
(define (xexpr-attributes xe)
  (local ((define optional-loa+content (rest xe)))
    (cond
      [(empty? optional-loa+content) '()]
      [else (local ((define possible-loa (first optional-loa+content)))
              (if (list-of-attributes? possible-loa)
                  possible-loa
                  '()))])))

;; LoA-or-LoX -> Boolean 
;; is the given value a list of attributes? 
(define (list-of-attributes? x)
  (cond
    [(empty? x) true]
    [else (local ((define possible-attribute (first x)))
            (cons? possible-attribute))]))

;; Xexpr -> [Listof Xexpr]
;; retrieve the content of an Xexpr 
(define (xexpr-content xe)
  (local ((define optional-loa+content (rest xe)))
    (cond
      [(empty? optional-loa+content) '()]
      [else (if (list-of-attributes? (first optional-loa+content))
                (rest optional-loa+content)
                optional-loa+content)])))

(define MISSING "missing attribute")

;; [Listof Attribute] -> [Maybe String]
;; retrieve a specific attribute value from an XML element
(define (attribute-value al a) 
  (local ((define r (assq a al)))
    (if (boolean? r) #f (second r))))