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