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