source 2007 Bernstein--Lange parameter ccd2 assume ccd2 = 2 c c d assume Z1 = 1 compute A = X1^2 compute B = Y1^2 compute C = A+B compute D = A-B compute E = (X1+Y1)^2-C compute Z3 = c D E compute X3 = C D compute Y3 = E (C-ccd2)