source 2007 Bernstein--Lange
assume X2 = 1
compute A = Z1 Z2
compute B = d A^2
compute D = Y1 Y2
compute E = X1 D
compute F = E-B
compute G = E+B
compute H = X1-D
compute I = X1 Y2+Y1
compute X3 = c G H
compute Y3 = c F I
compute Z3 = A H I