source 2007 Bernstein--Lange
parameter ccd2
assume ccd2 = 2*c*c*d
compute A = X1^2
compute B = Y1^2
compute C = A+B
compute D = A-B
compute E = (X1+Y1)^2-C
compute Z3 = c D E
compute X3 = C D
compute Y3 = E (C-ccd2 Z1^2)
