source 2008 Sutherland assume ZZ2 = 1 assume ZZZ2 = 1 compute U2 = X2 ZZ1 compute S2 = Y2 ZZZ1 compute P = U2-X1 compute R = S2-Y1 compute PP = P^2 compute PPP = P PP compute Q = X1 PP compute X3 = R^2-PPP-2 Q compute Y3 = R (Q-X3)-Y1 PPP compute ZZ3 = ZZ1 PP compute ZZZ3 = ZZZ1 PPP