Problem 5.   Your manager just came home from a consulting visit with a client. This (new) client keeps around non-empty lists of strings. Your company sold this client a ``sentence packaging'' program, which concatenates a list of strings into a single string. The original strings are separated via commas in the ``packaged string.'' Develop the program.

;; A Los is one of:
;; -- (cons String empty)
;; -- (cons String Los)

Solution:

;; pack : Los -> String  [PT 1]
;; concatenate the strings on alos;
;; separate them via commas [PT 1]
(define (pack alos)
  (cond ;; [PT 3, two for cond lines,
    ;; one for (empty? (rest ...))]
    [(empty? (rest alos)) (first alos)]
    ;; [PT 3, one for correctness,
    ;; two for "template" expressions ]
    [else (string-append
	    (first alos) "," (pack (rest alos)))]))

;; Examples/Tests: [PT 2, for correct tests/examples]
(string=? (pack (cons "hello" (cons "world" empty)))
          "hello,world")
(string=? (pack (cons "friedman" (cons "dan" empty)))
          "friedman,dan")