fe25519_sub
static void fe25519_sub(fe25519 *r, const fe25519 *x, const fe25519 *y);
fe25519_sub(&a, &r->y, &r->x); /* A = (Y1-X1)*(Y2-X2) */
fe25519_sub(&t1, &q->y, &q->x);
fe25519_sub(&e, &b, &a); /* E = B-A */
fe25519_sub(&f, &d, &c); /* F = D-C */
fe25519_sub(&a, &p->y, &p->x); /* A = (Y1-X1)*(Y2-X2) */
fe25519_sub(&t, &q->y, &q->x);
fe25519_sub(&r->x, &b, &a); /* E = B-A */
fe25519_sub(&r->t, &d, &c); /* F = D-C */
fe25519_sub(&r->x, &r->x, &a);
fe25519_sub(&r->x, &r->x, &b);
fe25519_sub(&r->t, &r->z, &c);
fe25519_sub(&r->y, &d, &b);
fe25519_sub(&num, &num, &r->z); /* x = y^2-1 */
fe25519_sub(r, r, &t);