source 2008 Hisil--Wong--Carter--Dawson, http://eprint.iacr.org/2008/522, Section 3.3
compute A = X1^2
compute B = Y1^2
compute C = 2 Z1^2
compute D = a A
compute E = (X1+Y1)^2-A-B
compute G = D + B
compute F = G - C
compute H = D - B
compute X3 = E F
compute Y3 = G H
compute T3 = E H
compute Z3 = F G