# oval(rx,ry,x0,y0) is oval with radiu rx and ry at (x0,y0);


oval(rx_):=(x/rx)^2+(y/rx)^2-1;
oval(rx_,ry_):=(x/rx)^2+(y/ry)^2-1;
oval(rx_,ry_,x0_,y0_):=(x-x0)^2/rx^2+(y-y0)^2/ry^2-1;