source 2005 Doche--Lange, plus common-subexpression elimination
compute A = Y1 Z2 + Z1 Y2
compute B = X1 Z2 + Z1 X2
compute C = B^2
compute D = Z1 Z2
compute E = B C
compute F = (A^2 + A B + a2 C) D + E
compute X3 = B F
compute Y3 = C (A X1 + Y1 B) Z2 + (A + B) F
compute Z3 = E D