// crosshairs
s = scale(-1.1, 1.1); 
plot cartesian(s, 0); 
plot cartesian(0, s); 

// arrow arc 
r = s2;
angle = 2*pi * (1 - s1);
plot polar(r, angle*t); 

// arrow head 
tip = polar(r, angle);
plot tip + polar(.1*t, angle - pi/4);
plot tip + polar(.1*t, angle - 3/4 * pi); 

// stone
plot polar(1, angle) + sling(.1, 1);

// sling strap
plot polar(t, angle);
