gf_sub
gf_sub(e->y, d->n->b, d->n->a);
gf_sub(e->x, n->b, n->a);
gf_sub(a, b, a);
gf_sub(y, y, u);
gf_sub(z, t, x);
gf_sub(t, t, z);
gf_sub(p->z, ONE, p->x); /* num = 1-y^2 */
gf_sub(p->t, ONE, p->t); /* denom = 1-dy^2 or 1-d + dy^2 */
gf_sub(b, b, d);
gf_sub(p->t, a, c);
gf_sub(a, p->z, d);
gf_sub(b->n->a, a->y, a->x);
gf_sub(c, a, b);
gf_sub(c, ZERO, c);
gf_sub(y, ZERO, x);
void gf_sub(gf out, const gf a, const gf b);