;; A Queue is one of
;; - '()
;; - (cons SchemeValue Queue)
;;
;; interpretation: 
;; - the empty list is an empty queue
;; - a non-empty list L is a queue whose head is (car L) and 
;;   whose tail is (cdr L)

;; empty : -> Queue
(define empty (lambda ()  '()))

;; snoc : Queue SchemeValue -> Queue
(define snoc
  (lambda (q v) ; [[ The body could also be written as (append q (list v)) ]]
    (cond 
     ((null? q) (cons v '()))
     (else (cons (car q)
                 (snoc (cdr q) v))))))

;; isEmpty: Queue -> Boolean
(define isEmpty (lambda (q) (null? q)))

;; head : Queue -> SchemeValue
(define head (lambda (q) (car q)))

;; tail : Queue -> Queue
(define tail (lambda (q) (cdr q)))