gf_isr
succ &= gf_isr(p->t, p->x); /* 1/sqrt(num * denom) */
ret = gf_isr(t2, t1); /* +-1/sqrt(o^2) = +-1/o */
mask_t gf_isr(gf a, const gf x); /** a^2 x = 1, QNR, or 0 if x=0. Return true if successful */