layer.placeBehind(layer)

Places this layer behind another layer. This changes the layer.index property for at least one of the layers. This method only works on layers that have the same parent, or no parent at all.
译：把这层放在另一层后面。这改变了layer.index特性至少一层。此方法只适用于具有相同父或根本没有父的层。

Arguments（参数）

layer — A layer object.译： 一层对象

layerA = new Layer
layerB = new Layer

# Draw layerB on top 
layerA.placeBehind(layerB)