source 2005 Doche--Lange, following Lopez-Dahab appliesto lopezdahab-0 parameter sqrta6 assume sqrta6^2 = a6 compute A = X1^2 compute B = sqrta6 Z1^2 compute C = X1 Z1 compute Z3 = C^2 compute X3 = (A+B)^2 compute Y3 = (B C + (Y1 + B) (A + B))^2