source 1998 Cohen--Miyaji--Ono "Efficient elliptic curve exponentiation using mixed coordinates", formula (4), plus common-subexpression elimination
compute w = a Z1^2+3 X1^2
compute s = Y1 Z1
compute ss = s^2
compute sss = s ss
compute R = Y1 s
compute B = X1 R
compute h = w^2-8 B
compute X3 = 2 h s
compute Y3 = w(4 B-h)-8 R^2
compute Z3 = 8 sss