;; ticketnum: number
;; state variable that stores the last ticket number handed out
(define ticketnum 0)

;; get-ticket:  -> number
;; produces the next ticket number
;; EFFECT:  increments ticketnum, or restarts it at 1
(define (get-ticket)
  (begin
    (cond [(= 4 ticketnum) (set! ticketnum 1)]
          [else (set! ticketnum (+ 1 ticketnum))])
    ticketnum))

;;------------------------------------------------------------------------

;; an account is a struct
;; (make-account number number)
(define-struct account (acctnum balance))

;; a list-of-account is either
;; empty, or 
;; (cons account list-of-account)

;; Citibank:  list-of-account
;; state variable that remembers the current contents of each bank account
(define Citibank (list (make-account 1 500)
                       (make-account 2 1000)
                       (make-account 3 10)))


;; add-account:  number number  -> void
;; consumes an acount number and amount, and adds a new account
;; EFFECT:  changes the number of accounts in Citibank
(define (add-account acctnum amt)
  (set! Citibank (cons (make-account acctnum amt) Citibank)))


;; remove-account:  number -> void
;; removes the account with the given account number
;; EFFECT: removes an account from Citibank
(define (remove-account acctnum)
  (set! Citibank (remove-from-list acctnum Citibank)))

;; remove-from-list: number list-of-account -> list-of-account
;; consumes a list of accounts and an account number and produces a list of accounts with the named account removed
(define (remove-from-list acctnum aloa)
  (cond [(empty? aloa)  (error "no such account")  ]
        [(cons? aloa)
         (cond [(= acctnum (account-acctnum (first aloa))) (rest aloa)]
               [else (cons (first aloa) (remove-from-list acctnum (rest aloa)))])]))