code: function(isBaseLayer) {
          if (isBaseLayer != this.isBaseLayer) {
              this.isBaseLayer = isBaseLayer;
              if (this.map != null) {
                  this.map.events.triggerEvent("changebaselayer", {
                      layer: this
                  });
              }
          }
      }
docs: APIMethod: setIsBaseLayer
  
  Parameters:
  isBaseLayer - {Boolean}