anchor.diff:      document.selection.empty();
     }
   };
  +
  +var emptyLayer = new google.maps.ImageMapType({
  +  tileSize: new google.maps.Size(256, 256),
  +  getTileUrl: function(tile, zoom) { }
  +});
   
   // extend parts of the Google Marker class
   google.maps.Marker.prototype.observation_id = null;
  
positive.code:     // we need to secure the layerID so that we can add the layer in the proper
      // places when the user selects it. That is why the layer is added
      // above and immediately set to null here.
      this.overlayMapTypes.setAt( layerID - 1, null);
    }
    if( this._overlayControl ) {
      this._overlayControl.addWindshaftOverlayControl(
  
negative.code:   checkbox.change( function( ) {
      var checked = this.checked;
      _.each( layers, function( layerData ) {
        map.overlayMapTypes.setAt( layerData.layerID - 1, checked ? layerData.layer : emptyLayer);
      });
      // if this checkbox belongs to a taxon, make sure that if this is the
      // last in the set unchecked, the taxon is also unchecked. And as soon
  