function arch(a,b,theta1,theta2,r1,r2,ring_color)

% Adds an arch with center (a,b), inner radius r1, and outer radius r2 to the current figure.
% The arch is the set of all points of the form (a+r*cos(theta),b+r*sin(theta)) where
% r1 <= r <= r2 and theta1 <= theta <= theta2, with theta1 and theta2 in radians.
% The color of the displayed arch is prescribed by ring_color, a 3-vector encoding the rgb triple.

% Build arch in .01 radian increments, being sure to include the endpoint

theta = [theta1:.01:theta2 theta2];
n = length(theta);

% Counter-clockwise around the outside

outx = a + r2*cos(theta);
outy = b + r2*sin(theta);

% Clockwise around the inside

inx = a + r1*cos(theta(n:-1:1));
iny = b + r1*sin(theta(n:-1:1));

% Draw the ring

fill([outx inx], [outy iny], ring_color);