source 2008 Sutherland compute U1 = X1 ZZ2 compute U2 = X2 ZZ1 compute S1 = Y1 ZZZ2 compute S2 = Y2 ZZZ1 compute P = U2-U1 compute R = S2-S1 compute PP = P^2 compute PPP = P PP compute Q = U1 PP compute X3 = R^2-PPP-2 Q compute Y3 = R (Q-X3)-S1 PPP compute ZZ3 = ZZ1 ZZ2 PP compute ZZZ3 = ZZZ1 ZZZ2 PPP