; create prime list from 2 thru n.
; primetest will return an integer x if x is prime; otherwise it returns nil.
; mapcar'ing primetest to integer list, then deleting the nils, results in
; desired answer.

(defun primes (n)
   (cond ((<= n 1) nil)
	 ((equal n 2) '(2))
	 (t (cons 2 (filternils (mapcar 'primetest (nlist 3 n)))))
   )
)


(defun nlist (a b)
     (cond ((>= a b) (list b))
	   (t  (cons a (nlist (1+ a) b) ) )
     )
)

;;; simple iterative testing for a prime number: mod all integers between 2 and sqrt(n)

(defun primetest (n)
   (let ((end (ceiling (sqrt n))) (cnt 2) )
      (loop
	 (cond ((zerop (mod n cnt)) (return nil))
	       ((equal cnt end) (return n))
         )
	 (setq cnt (1+ cnt))
       )  
   )  
)

;;; delete nils from list l

(defun filternils (l)
   (cond ((null l) nil)
	 ((null (car l)) (filternils (cdr l)))
	 (t (cons (car l) (filternils (cdr l))))
   )
)