title: Google Maps polygon/overlay - deleting
body: <p>As a follow-on to a previous question... I am having no luck deleteing 
  an overay that I have drawn on the map, so that I can start over.  </p>
  
  <p>My sequence:  </p>
  
  <p>1) draw the map<br>
  2) the user selects a city<br>
  3) I make an ajax call to get the boundary coordinates for 
     the selected city, and within the ajax success block, I 
     draw a boundary around that city - to be complete, I draw 
     a polygon on the map that completely covers the viewport, 
     except for a "hole" punch in it that exposes the city.  </p>
  
  <p>Great, so far  </p>
  
  <p>4) The user selects a new city<br>
  5) Same ajax call, but it starts with trying to wipe out the
     overlay by setting the "map" to null.  </p>
  
  <p>That doesn't work. The new city has a border around it, but
     the overlay has become darker, and the original "hole" is 
     still poked through.</p>
  
  <p>I'm using:   negativeSpacePolygon.setMap( null );  </p>
  
  <p>to try and clear out the overlay. What am I missing?</p>