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