clean-up

  ;; [Listof Posn] Number Number Number Number 
  ;; -> 
  ;; [Listof Posn]
  ;; remove specks that are inside of 
  ;; the specified boundaries 
  (define (clean-up lop lft rgt top bot)
    (cond
      [(empty? lop) empty]
      [else (local ((define fst (first lop))
                    (define rst 
                      (clean-up 
                        (rest lop) lft rgt top bot)))
              (cond [(and (<= lft (posn-x fst) rgt)
                          (<= top (posn-y fst) bot))
                     rst]
                    [else (cons fst rst)]))]))

  ;; TESTS
  (equal? (clean-up (list (make-posn 10 10)
                          (make-posn 100 50)
                          (make-posn 20 10))
                    10 20 10 20)
          (list (make-posn 100 50)))

Solution:

;; [PTS 2: 1 for filter, 
;;         1 for correctness (function, not)]
(define (clean-up lop lft rgt top bot)
  (local ((define (outside? fst)
            (not (and (<= lft (posn-x fst) rgt) 
                      (<= top (posn-y fst) rgt)))))
    (filter outside? lop)))