A layered object represents data that should be plotted in
  successive layers, for example, a background and a foreground.
  See layered.
The function [.layered
  extracts a designated subset of a layered object.
  It is a method for [ for the
  class "layered".
The functions [<-.layered and [[<-.layered
  replace a designated subset or designated entry of the object by new
  values. They are methods for [<- and [[<-
  for the "layered" class.
The index i specifies which layers will be retained.
  It should be a valid subset index for the list of layers.
The index j will be applied to each layer. It is typically
  a spatial window (class "owin") so that each of the layers
  will be restricted to the same spatial region.
  Alternatively j may be any subset index
  which is permissible for the "[" method for each of the layers.