source 2009 Bernstein--Kohel--Lange
compute A = X1 Z2
compute B = Z1 Z2
compute C = Y1 X2
compute D = Y1 Y2
compute E = Z1 Y2
compute F = a X1 X2
compute X3 = A B - C D
compute Y3 = D E - F A
compute Z3 = F C - B E