[plt-scheme] Htdp chapter 12 insertion sort

I have a question about the insertion sort algorithm implemented in chapter
12.The figure 33 explains

;; sort : list-of-numbers  ->  list-of-numbers (sorted)
;; to create a list of numbers with the same numbers as
;; alon sorted in descending order
(define (sort alon)
  (cond
    [(empty? alon) empty]
    [(cons? alon) (insert (first alon) (sort (rest alon)))]))- what is
cons? alon here , is it required there?


Thanks

Aditya
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