This is the plot method for the class "listof". An object of class "listof" (defined in the base R package) represents
a list of objects, all belonging to a common class.
The base R package defines a method for printing these objects,
print.listof, but does not define a method for plot.
So here we have provided a method for plot.
In the spatstat package, the function
density.splitppp produces an object of
class "listof", essentially a list of pixel images.
These images can be plotted in a nice arrangement
using plot.listof. See the Example.