‘And how do we find the Index File?’ A silly thought came into Tegan’s head. ‘Of course, if we had the Index File we could look it up in the Index File under Index File.’ The tension was getting to her; she was thinking and talking nonsense.

But Nyssa took it in her stride. Without pausing at her work at the keyboard she said: ‘Well done. You’ve just discovered recursion.’ Tegan was surprised to be taken so seriously, but Nyssa went on to explain that recursion was a powerful method used to solve some kinds of mathematical problems. ‘It’s when procedures fold back on themselves.’

‘Oh, I don’t understand anything about maths,’ Tegan said. She remembered school exams, and how the wretched figures never seemed to stay still on the paper in front of her.