The function generate consumes a natural number (n) and creates a list of that many ``diagonalized'' Posns:

;; generate : NaturalNumber -> Listof[Posn]
;; creates a list of that many ``diagonalized'' posns
(define (generate n) 
  (cond
    [(zero? n) empty]
    [else (cons (make-posn n n) (generate (sub1 n)))]))

;; Examples/tests: 
(equal?
  (generate 2)
  (list (make-posn 2 2) (make-posn 1 1)))
(equal?
  (generate 3) 
  (list (make-posn 3 3) (make-posn 2 2) (make-posn 1 1)))

Solution

(define (generate n) ;; [PT 2 -- 1 for helper, 1 for (- n i)]
 (local ((define (diag i) (make-posn (- n i) (- n i))))
   (build-list n diag))) ;; [PT 1 -- for build-list]