source 2008 Bernstein--Lange--Rezaeian-Farashahi "Binary Edwards Curves", plus specialization to Z1=1, plus simplification assume Z1 = 1 compute A = X1^2 compute B = Y1^2 compute Z3 = A compute ZZ3 = A^2 compute X3 = ZZ3 + a6 compute XZ3 = X3 Z3 compute Y3 = B (B + X3 + Z3) + a6 ZZ3 + XZ3