code: function(layers) {
          if(OpenLayers.Util.isArray(layers)) {
              this.layers = layers;
              this.layer = new OpenLayers.Layer.Vector.RootContainer(
                  this.id + "_container", {
                      layers: layers
                  }
              );
          } else {
              this.layer = layers;
          }
      }
docs: Method: initLayer
  Assign the layer property. If layers is an array, we need to use
  a RootContainer.
  
  Parameters:
  layers - {<OpenLayers.Layer.Vector>}, or an array of vector layers.