source 2008 Bernstein--Lange
assume Z1 = 1
assume Z2 = 1
compute A = Y1 + Y2
compute B = X1 + X2
compute AB = A + B
compute C = B^2
compute Z3 = B C
compute F = A AB + a2 C + Z3
compute X3 = B F
compute Y3 = C (A X1 + B Y1) + AB F