No go. There was an invisible wall behind me now, too. And one to the right and to the left when I checked. It was like I was in an invisible and slightly undersized phone booth. For a couple of seconds I did the old street-mime shtick, palms pressing flat against unseen walls. Then I cringed and covered my ears as a high-pitched siren shrieked from somewhere behind my left shoulder. Frag, why not? Invisible walls—why not an invisible burglar alarm, too?