‘No one is laughing at you. This is cracking ciphers, not spreading nets to catch fish. This was an unusually large exercise and our comrades all worked extra hard on it, and so you might expect that they are extremely disappointed it didn’t work out. But they all understand what you are going through, because they get to see Jiang Nan wandering around outside their windows every day. They see him every day, they think about it every day – that even though cracking ciphers is an intangible achievement, it can make you pay a truly terrible price.’

She was deeply moved. ‘I don’t know what to say … that’s terribly kind of you. Thank you.’

I smiled. ‘Thank you for your praise: it is a great compliment.’

She said seriously, ‘It’s true: I have a lot of respect for you. You don’t seem to mind when things go wrong. I simply am not like that – I can’t do it.’

I tried to cheer her up. ‘You mustn’t let this get you down. This isn’t a failure, this is just one of those setbacks that any cryptographer has to face. Cracking a cipher isn’t like doing a crossword, where a moment of inspiration gets you the answer.’

Her eyes sparkled and she put her hand down lightly on my shoulder. ‘I understand. Don’t worry, this won’t get me down. Before leaving Beijing I went off to bow to the statue of the god of mathematics, Zu Chongzhi, and made a wish. I believe that the gods will protect us.’

I took her hand with the intention of getting it off my shoulder, but she grabbed hold of me and said seriously, ‘Zaitian, I know that you don’t dare to love me, and so I have been doing my best to forget you, to move on, but it’s not working. What am I supposed to do?’