source 1998 Cohen--Miyaji--Ono "Efficient elliptic curve exponentiation using mixed coordinates", formula (5)
unified
compute U1 = X1 Z2^2
compute U2 = X2 Z1^2
compute S1 = Y1 Z2^3
compute S2 = Y2 Z1^3
compute H = U2-U1
compute r = S2-S1
compute X3 = r^2-H^3-2 U1 H^2
compute Y3 = r (U1 H^2-X3)-S1 H^3
compute Z3 = Z1 Z2 H
