source 2007 Bernstein--Lange
assume Z1 = 1
compute XX = X1^2
compute w = a+3 XX
compute Y1Y1 = Y1^2
compute R = 2 Y1Y1
compute sss = 4 Y1 R
compute RR = R^2
compute B = (X1+R)^2-XX-RR
compute h = w^2-2 B
compute X3 = 2 h Y1
compute Y3 = w(B-h)-2 RR
compute Z3 = sss