# ring(r1,r2) is ring with radiu r1 and r2 at (x0,y0);


ring(r1_,r2_):=abs((x^2+y^2-r1^2)*(x^2+y^2-r2^2))+(x^2+y^2-r1^2)*(x^2+y^2-r2^2);
ring(r1_,r2_,x0_,y0_):=abs(((x-x0)^2+(y-y0)^2-r1^2)*((x-x0)^2+(y-y0)^2-r2^2))+((x-x0)^2+(y-y0)^2-r1^2)*((x-x0)^2+(y-y0)^2-r2^2);