source 2006 Doche--Icart--Kohel "Efficient scalar multiplication by isogeny decompositions", page 199, top display, plus Z2=1, plus simplification, plus projectification compute D = Y2/Z2^3-Y1/Z1^3 compute Z3 = X2/Z2^2-X1/Z1^2 compute X3 = D^2-Z3^3-3 a Z3^2-2 Z3^2 X1/Z1^2 compute Y3 = D(Z3^2 X1/Z1^2-X3)-Z3^3 Y1/Z1^3 compute ZZ3 = Z3^2