With special thanks to all who gave me encouragement – in particular, David Miller and all at Rogers, Coleridge and White, and Michael Fishwick, who saw the point of this before it existed and without whom it probably wouldn’t. Thanks to all at Bloomsbury, Kathy Fry and particular props to Colin Midson, Ruth Logan, Sophia Martelli and Alex Goodwin. And thanks to Umar Salam, for nurturing my maths-envy.