docs: function(layer, position, bounds, url, size, options) {
          this.layer = layer;
          this.position = position.clone();
          this.setBounds(bounds);
          this.url = url;
          if (size) {
              this.size = size.clone();
          }
  
          //give the tile a unique id based on its BBOX.
          this.id = OpenLayers.Util.createUniqueID("Tile_");
  
          OpenLayers.Util.extend(this, options);
  
          this.events = new OpenLayers.Events(this);
          if (this.eventListeners instanceof Object) {
              this.events.on(this.eventListeners);
          }
      }