# disc(r,x0,y0) is disc with radiu r at (x0,y0);


disc(r_):=abs(x^2+y^2-r^2)+(x^2+y^2-r^2);
disc(r_,x0_,y0_):=abs((x-x0)^2+(y-y0)^2-r^2)+((x-x0)^2+(y-y0)^2-r^2);