source 1998 Cohen--Miyaji--Ono "Efficient elliptic curve exponentiation using mixed coordinates", formula (10), plus common-subexpression elimination compute XX = X1^2 compute YY = Y1^2 compute U = 8 YY^2 compute S = 4 X1 YY compute M = 3 XX + T1 compute X3 = M^2 - 2 S compute Y3 = M(S-X3)-U compute Z3 = 2 Y1 Z1 compute T3 = 2 U T1