code: function(px) {
          var newPx = this.clone();
          if (px) {
              newPx = this.add(px.x, px.y);
          }
          return newPx;
      }
docs: APIMethod: offset
  
  Parameters
  px - {<OpenLayers.Pixel>|Object} An OpenLayers.Pixel or an object with
  a 'x' and 'y' properties.
  
  Returns:
  {<OpenLayers.Pixel>} A new Pixel with this pixel's x&y augmented by the
  x&y values of the pixel passed in.