source 2007 Bernstein--Lange
compute B = (X1+Y1)^2
compute C = X1^2
compute D = Y1^2
compute E = C+D
compute H = (c Z1)^2
compute J = E-2 H
compute X3 = c (B-E)J
compute Y3 = c E(C-D)
compute Z3 = E J