source 2005 Doche--Lange / Higuchi Takagi; modified to compute and use Z^2
compute A = X1 Z2
compute B = X2 Z1
compute C = A^2
compute D = B^2
compute E = A + B
compute F = C + D
compute G = Y1 ZZ2
compute H = Y2 ZZ1
compute I = G + H
compute J = I E
compute Z3 = F Z1 Z2
compute ZZ3 = Z3^2
compute X3 = A (H + D) + B (C+G)
compute Y3 = (A J + F G) F + (J + Z3) X3