source 2007 Bernstein--Lange
assume Z2=1
compute Z1Z1 = Z1^2
compute U2 = X2 Z1Z1
compute S2 = Y2 Z1 Z1Z1
compute H = U2-X1
compute HH = H^2
compute I = 4 HH
compute J = H I
compute r = 2 (S2-Y1)
compute V = X1 I
compute X3 = r^2-J-2 V
compute Y3 = r (V-X3)-2 Y1 J
compute Z3 = (Z1+H)^2-Z1Z1-HH