docs: function() {
          var origin = this.tileOrigin;
          if (!origin) {
              var extent = this.getMaxExtent();
              var edges = ({
                  "tl": ["left", "top"],
                  "tr": ["right", "top"],
                  "bl": ["left", "bottom"],
                  "br": ["right", "bottom"]
              })[this.tileOriginCorner];
              origin = new OpenLayers.LonLat(extent[edges[0]], extent[edges[1]]);
          }
          return origin;
      }