docs: function (bounds) {
          var xyz = this.getXYZ(bounds);
          var url = this.url;
          if (OpenLayers.Util.isArray(url)) {
              var s = '' + xyz.x + xyz.y + xyz.z;
              url = this.selectUrl(s, url);
          }
          
          return OpenLayers.String.format(url, xyz);
      }