source 2006 Doche--Icart--Kohel "Efficient scalar multiplication by isogeny decompositions", page 196, bottom display
parameter a16
assume a16 = 16 a
compute A = X1^2
compute B = X1^2-a16 Z1^2
compute YT = Y1 B
compute X3 = B^2
compute Z3 = 4 Y1^2
compute C = X1^2 a Z1^2
compute D = Z3^2
compute E = a (Z3-4 C)
compute Y3 = YT (2 X3+E+256 C)
compute ZZ3 = D