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


Sierpinski(n)={
  my(s=2^n-1);
  forstep(y=s,0,-1,
    for(i=1,y,print1(" "));
    for(x=0,s-y,
      print1(if(bitand(x,y)," ","*"))
    );
    print()
  )
};
Sierpinski(4)