Xnleft
debunk(xp, sp, Xnleft(*xs, xp));
while ((n = shf_read(xp, Xnleft(xs, xp), shf)) > 0) {
if (Xnleft(xs, xp) <= 0)
char *p = shf_getse(xp, Xnleft(s->xs, xp), s->u.shf);