code for paper

> pl1 := plot({seq([[i,-10],[i,10]],i=-10..10),
seq([[-10,i ],[10,i]],i=-10..10)}, color=blue):

> pl2 := plot({[[0,-10],[0,10]],[[-10,0],[10,0]]},
color=blue,thickness=3):

> pap :=plots[display]([pl1,pl2],axes=none,
scaling=constrained):
pap;