‘The semiprime? Oh, up to 617 digits. Factoring them is an NP class problem.’

‘A what?’

‘Like I told you in Paris. A nondeterministic …’ He cocked his head, patronizingly. ‘A very complex problem that professors like Rachel can work on for their whole careers.’

‘OK,’ I said. ‘Let’s start at the beginning. How do you solve problems? Easy problems. One I could understand. Like …’ I tried and failed to think of an easy maths question, ‘a basic equation, or something.’

Baaz eyed me suspiciously. ‘Simplify,’ he replied. ‘Get rid of all the clutter, so you can see exactly what the definites are. And then …’ He paused.

I nodded at him. ‘Go on.’

‘And then you have to choose how you’re going to do it – by brute force, trying every possible answer consecutively, or by using a shortcut, an algorithm.’ He picked up another piece of the dessert. ‘Why, how do you solve problems?’

‘By eliminating their source.’

I stopped fiddling with the packet of Marlboros and pulled one out. I flipped open the book of matches that Talia had given me and tore a sulphur-tipped strip of card free of the base, and then looked again. There were only two other matches missing.

‘That’s strange,’ I said under my breath, closing the cover. On the front was a black 7 logo stamped over a yellow flame. Probably some spook bar in Tel Aviv. I opened it again and pulled the matches away from the back cover. Printed in neat handwriting was an Israeli cell phone number. I put my hand into the jacket pocket I’d kept the cigarettes in. Empty. I tried the other one, and there it was: an identical book of matches, only with half the matches missing and no phone number – the book Talia had given me. I stood up and left a hundred-shekel note on the table.

‘What’s the matter?’ Baaz asked, stuffing the last piece of baklava into his mouth.

‘Nothing,’ I said. ‘Let’s go.’

‘Ken, rega.’

The bolts of the door scraped back again, the uncomfortable screech of metal dragging on metal. Moshe Mendel Katz’s face appeared with a breath of turpentine in the opening; by his waist, the close-cropped head of little Binyomen.

‘There was another question,’ I said, ‘that I wanted to ask.’ I stepped forward and pushed the door. He stopped it with his foot.

‘I’m sorry, but …’

I pushed harder and put my right shoulder into the door.

‘Don’t worry,’ I said, ‘it won’t take long.’ A shadow passed over the child’s face. He backed away. Moshe caught his right arm by the wrist. As he did so, I shoved past them and back into the dark cavern of the studio warehouse. Baaz stepped forward, too, into the doorway, blocking the glare of the day outside. Moshe reached for the light switch. I reached for the SIG. He paused.