docs: function(evt) {
          if (this.active && !evt.aborted &&
                  evt.tile instanceof OpenLayers.Tile.Image &&
                  evt.tile.url.substr(0, 5) !== 'data:') {
              this.cache({tile: evt.tile});
              delete OpenLayers.Control.CacheWrite.urlMap[evt.tile.url];
          }
      }