source 2005 Dimitrov--Imbert--Mishra, plus common-subexpression elimination
compute ZZ = Z1^2
compute YY = Y1^2
compute C = 2 YY
compute M = 3 X1^2+a ZZ^2
compute E = 6 X1 C-M^2
compute EE = E^2
compute T = 2 C^2
compute U = M E-T
compute U4 = 4 U
compute X3 = X1 EE-C U4
compute Y3 = Y1 (U4 (T-U)-E EE)
compute Z3 = Z1 E
