source 2007 Bernstein--Lange assume Z1=1 assume Z2=1 compute H = X2-X1 compute HH = H^2 compute I = 4 HH compute J = H I compute r = 2 (Y2-Y1) compute V = X1 I compute X3 = r^2-J-2 V compute Y3 = r (V-X3)-2 Y1 J compute Z3 = 2 H