“…The direction of the motion in the chain Phi can be defined using the halfplane {t I dist(t, pj) = dist(t, pi) and dist(t, pj) < dist(t, pi)} fore 54 eo and the halfplane n+ fl {tj dist(t, pi) = dist(t, Pk)} for e = el 2. The edge E' does not contain the point r if and only if the line passing through endpoints of E' separates the points r and (p + p,,)/2 in the bisector y(j, s).…”