code: func (f *Fpdf) BeginLayer(id int) {
  	f.EndLayer()
  	if id >= 0 && id < len(f.layer.list) {
  		f.outf("/OC /OC%d BDC", id)
  		f.layer.currentLayer = id
  	}
  }
docs: // BeginLayer is called to begin adding content to the specified layer. All
  // content added to the page between a call to BeginLayer and a call to
  // EndLayer is added to the layer specified by id. See AddLayer for more
  // details.