#| 
   A PR record is
    - (define-struct PR (name exchange profit))

   A list-of-PRs is either 
    - empty 
    - (cons p lor) where p is a PR record and lor is a list-of-PRs. 

   (define (LOPR-template alopr)
     (cond
       [(empty? alopr) ...]
       [else ... (first alopr) ... (LOPR-template (rest alopr)) ...]))
             ;;  ^^^ a PR-record   ^^^ possible natural recursion
|#

;; project : list-of-PRs -> list-of-symbols
;; return the names of all PR records with profit between 10 and 20 (incl)
(define (project alopr)
  (names (okay-prs alopr)))

;; names : list-of-PRS -> list-of-symbols
;; return the names of all the PR records
(define (names alopr)
  (cond
    [(empty? alopr) empty]
    [else (cons (PR-name (first alopr)) (names (rest alopr)))]))

;; okay-prs : list-of-PRs -> list-of-PRs
;; return all PR records with profit between 10 and 20 (incl)
(define (okay-prs alopr)
  (cond
    [(empty? alopr) empty]
    [else (cond
            [(okay? (first alopr))
             (cons (first alopr) (okay-prs (rest alopr)))]
            [else (okay-prs (rest alopr))])]))

;; okay? : PR -> boolean
;; determine whether a-PR's profit is between 10 and 20 (incl)
(define (okay? a-PR)
  (<= 10 (PR-profit a-PR) 20))