source 2007 Bernstein--Lange
assume Z1=1
compute XX = X1^2
compute YY = Y1^2
compute YYYY = YY^2
compute S = 2((X1+YY)^2-XX-YYYY)
compute M = 3 XX+a
compute T = M^2-2 S
compute X3 = T
compute Y3 = M(S-T)-8 YYYY
compute Z3 = 2 Y1
