source 2006 Doche--Icart--Kohel "Efficient scalar multiplication by isogeny decompositions", page 198, middle display, plus common-subexpression elimination compute YY1 = Y1^2 compute A = (X1+3 ZZ1)^2 compute B = a ZZ1 A compute Xt = YY1+B compute Yt = Y1(YY1-3 B) compute Zt = X1 Z1 compute C = Zt^2 compute Ca = a C compute E = 3 Ca(4 Ca-9 C-Xt)^2 compute X3 = Yt^2-E compute Y3 = Yt(X3+4 E) compute Z3 = 3 Xt Zt compute ZZ3 = Z3^2