f25519_sub(&t1, &t0, &t1); /* t1 = J = F - 2 H */
f25519_mul(X, X, &t1); /* X = (B - C - D) J */
f25519_mul(Y, Y, &t0); /* Y = -F (E - D) */
f25519_sub(&t1, &t0, &t1); /* t1 = J = F - 2 H */
f25519_mul(X, X, &t1); /* X = (B - C - D) J */
f25519_mul(Y, Y, &t0); /* Y = -F (E - D) */