“What they are is a small tablet about six inches square, which has a screen in it. As you walk it shows a scrolling digital map of the area you’re in, telling you what each store you pass sells, who lives in what block, the whole works, updated by small beacons on every street corner. If you tap in a destination the screen shows you a red line to follow, and the tablet whispers at you to tell you when to make a turn.”
