# sphere(r,x0,y0) is sphere with radiu r at (x0,y0,z0);


sphere(r_):=(x^2+y^2+z^2-r^2);
sphere(r_,x0_,y0_,z0_):=((x-x0)^2+(y-y0)^2+(z-z0)^2-r^2);