gf_mulw
gf_mulw(c, b, TWISTED_D);
gf_mulw(p->t, p->x, EDWARDS_D); /* dy^2 */
gf_mulw(t1, t2, -EDWARDS_D); /* E*-d = a24*E */
gf_mulw(b->n->c, a->t, 2 * TWISTED_D);