// --------------
//  The G series
// --------------
height=400;
width =400;
clip=none;
do_background=yes;
clear_screen;
curve = -(y+x+x*y)^3+4*(1+x+y)*(y+x+x*y)*x*y-8*x^2*y^2;
draw_curve;
do_background=no;
curve = (y+x+x*y);
draw_curve;