code: func (f *Fpdf) AddLayer(name string, visible bool) (layerID int) {
  	layerID = len(f.layer.list)
  	f.layer.list = append(f.layer.list, layerType{name: name, visible: visible})
  	return
  }
docs: // AddLayer defines a layer that can be shown or hidden when the document is
  // displayed. name specifies the layer name that the document reader will
  // display in the layer list. visible specifies whether the layer will be
  // initially visible. The return value is an integer ID that is used in a call
  // to BeginLayer().