source 2007 Bernstein--Lange appliesto projective-3 compute w = 3(X1-Z1)(X1+Z1) compute s = 2 Y1 Z1 compute ss = s^2 compute sss = s ss compute R = Y1 s compute RR = R^2 compute B = 2 X1 R compute h = w^2-2 B compute X3 = h s compute Y3 = w(B-h)-2 RR compute Z3 = sss