Task
Produce an ASCII representation of a Sierpinski triangle of order   N.



Example
The Sierpinski triangle of order   4   should look like this:

                       *
                      * *
                     *   *
                    * * * *
                   *       *
                  * *     * *
                 *   *   *   *
                * * * * * * * *
               *               *
              * *             * *
             *   *           *   *
            * * * *         * * * *
           *       *       *       *
          * *     * *     * *     * *
         *   *   *   *   *   *   *   *
        * * * * * * * * * * * * * * * *



Related tasks
 Sierpinski triangle/Graphical for graphics images of this pattern. 
 Sierpinski carpet


(define (sierpinski n)
  (for-each
   (lambda (x) (display (list->string x)) (newline))
   (let loop ((acc (list (list #\*))) (spaces (list #\ )) (n n))
     (if (zero? n)
         acc
         (loop
          (append
           (map (lambda (x) (append spaces x spaces)) acc)
           (map (lambda (x) (append x (list #\ ) x)) acc))
          (append spaces spaces)
          (- n 1))))))