Moves the layer down the layer tree (towards the root layer) if the parent layer is not the root layer.
Examples
if(interactive()){
p <- l_plot()
g1 <- l_layer_group(p)
g2 <- l_layer_group(p, parent=g1)
l1 <- l_layer_oval(p, x=0:1, y=0:1, parent=g2)
l_layer_printTree(p)
l_layer_promote(p, l1)
l_layer_printTree(p)
l_layer_promote(p, l1)
l_layer_printTree(p)
}