source 2008 Giessmann assume Z2=1 compute T1 = Z1^2 compute T2 = T1 Z1 compute T1 = T1 X2 compute T2 = T2 Y2 compute T1 = X1-T1 compute T2 = T2-Y1 compute Z3 = Z1 T1 compute T4 = T1^2 compute T1 = T1 T4 compute T4 = T4 X1 compute X3 = T2^2 compute X3 = X3+T1 compute Y3 = T1 Y1 compute T1 = 2 T4 compute X3 = X3-T1 compute T4 = X3-T4 compute T4 = T4 T2 compute Y3 = T4-Y3