‘You know I can’t tell you that.’ The book would be collected from the stacks at 9 a.m. next day. ‘You may re-request it for yourself, and they can have it for one week. You’ll get it back on the morning of the eighth day.’

His fingers clenched the red spine. Back in the domed reading room, he darted his eyes around. Not since its publication had another person requested Uma Nova Luz Num Litoral Antigo. Now out of the blue someone wanted to read it.

A male assistant in a maroon cardigan pushed a loaded trolley across the carpet like a sleepwalker.

The drumming in his heart. The heels of his pursuers. Whoever had ordered up the book, they were not interested in Madrugada’s scholarship – of this he was sure – but in Marvar’s scrap of paper that Dyer had reinserted, moments before, between pages 223 and 224.

How quickly a man’s beard grows when he’s dead. Marvar’s algorithm was like that, it had taken on a supernatural life of its own. Dyer couldn’t leave it there.

Heavy-hearted, he returns to his chair with the book. Unlike the couple in the painting, he has no time, he has to act now. His fingers shake as he takes out and starts to reread the figures and letters, in the hand that wrote ‘BOOMERANG’ on his fridge.

Was any real purpose served by this yellow post-it note – beyond its use as a bookmark? Far simpler to rip it into a thousand pieces and scatter them on Port Meadow. It would remove the hazard of the note being found in his possession. Besides, who would know? He hadn’t admitted to anyone that he has this. And if he did remain under suspicion that he might be hiding something, it wouldn’t be for long; enough time would pass for everyone to forget about it. He would be free, safe.

The temptation to obliterate, as Marvar had obliterated his marks in the sand, was fierce. But even as Dyer yielded to it, he remembered Marvar’s searching eyes. Those eyes had taken Dyer out of himself into a world where suddenly nothing seemed impossible if it was to be done for another person in trouble. Irrespective of whether he believed in Marvar’s abracadabra, Dyer was bound by his obtuse sense of duty to a man who for some reason had touched him.

If Dyer was not to erase Marvar’s calculations, then the only alternative was to preserve them. He couldn’t risk putting the note back into his shoulder bag. Recent days had spelled out the very clear danger of having it on his person or in his house.

The most obvious immediate hiding place – until he reached a settled decision about what to do with it – was another book.

He returned to the main reading room.