// --------------
//  The G series
// --------------
height=400;
width =400;
clip=none;
clear_screen;
curve = 4*(y+x+x*y)^4*(1+x+y)*x*y
        -16*(1+x+y)*(y+x+x*y)*x^3*y^3
        -(y+x+x*y)^6-4*(y+x+x*y)^3*x^2*y^2
        +16*x^4*y^4;
draw_curve;