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