wnd_is_free
if (!wnd_is_free(wnd, op, 1))
if (wnd_is_free(wnd, bit + i, 1)) {
while (zlen > 1 && !wnd_is_free(wnd, zbit, zlen))
bool wnd_is_free(struct wnd_bitmap *wnd, size_t bit, size_t bits);