sample version of the USER routine, which can be used to
          specify a region which is not the unit hypercube.
        </li>
        <li>
          <a href = "user_circle_2d.m">user_circle_2d.m</a> 
          a sample version of the USER routine, which in this case,
          returns points in the unit circle, under a uniform density.
        </li>
        <li>
          <a href = "user_box_3d.m">user_box_3d.m</a> 
          a sample version of the USER routine, which in this case,
          returns p