Not practical for portable use. For portable use you need a battery, and for a linear yard of explosive you need some volts and amps. Tiny AA cells put out a weak volt and a half. Not enough, according to prevailing rules of thumb. A nine-volt battery is better, and for a decent kick you want one of the big square soup-can size cells sold for serious flashlights. Too big and too heavy for a pocket, hence the bag. The battery nestles in the bottom of the bag, wires come off it to the switch, then they head on out through an unobtrusive slit in the back of the bag, and then they loop up under the hem of the inappropriate garment.

Passenger number four was wearing a black canvas messenger bag, urban style, looped in front of one shoulder and behind the other, and hauled around into her lap. The way the stiff fabric bulged and sagged made it look empty apart from a single heavy item.

The train stopped at 28th Street. The doors opened. No one got on. No one got off. The doors closed and the train moved on.

Point eleven: hands in the bag.

Twenty years ago point eleven was a recent addition. Previously the list had ended at point ten. But things evolve. Action, and then reaction. Israeli security forces and some brave members of the public had adopted a new tactic. If your suspicions were aroused, you didn’t run. No point, really. You can’t run faster than shrapnel. What you did instead was grab the suspect in a desperate bear hug. You pinned their arms to their sides. You stopped them reaching the button. Several attacks were prevented that way. Many lives were saved. But the bombers learned. Now they are taught to keep their thumbs on the button at all times, to make the bear hug irrelevant. The button is in the bag, next to the battery. Hence, hands in the bag.

Passenger number four had her hands in her bag. The flap was bunched and creased between her wrists.

The train stopped at 33rd Street.