Develop the template for a function that consumes Packages.

  ;; profun : Package -> ???
  (define (profun p) ...)

Solution:

;; profun : Package -> ???
(define (profun p)
  (cond ;; [PT 3, for 3 cond lines]
    [(regular? p)
     .. (regular-zip p) ..]
    [(express? p)
     .. (express-zip p) .. (express-zap p) ..
     .. (express-zoom p)]
    [(overnight? p)
     .. (overnight-zip p) .. (overnight-zap p)]
;; [PT 3, for getting three selector expressions right]