function s = GetSine(P,Q,R)
% P, Q, and R are column 2-vectors.
% s is the sine of the angle obtained by rotating
% vector P-Q counter-clockwise so that it points in the
% direction of vector R-Q

u = P-Q; u = u/norm(u);
v = R-Q; v = v/norm(v);
s = u(1)*v(2) - u(2)*v(1);
