Animate 2020.
timeline.mergeLayers(startLayerNum:int,endLayerNum:int).
startLayerNum:It is an integer that specifies the starting layer index.
endLayerNum:It is an integer that specifies the ending layer index.
Layer Object of the merged layer.
Method; Merge the layers within specified range of indexes.
The following example merge layers from index 0 to 3.
fl.getDocumentDOM().getTimeline().mergeLayers(0,3);