The inner walls of the warehouse were covered with numbers. Equations as complex as a neural network had been scraped in the frost. At some point in the calculation the mathematician had changed from using numbers to using letters, and then letters themselves hadn't been sufficient; brackets like cages enclosed expressions which were to normal mathematics what a city is to a map.

They got simpler as the goal neared – simpler, yet containing in the flowing lines of their simplicity a spartan and wonderful complexity.

Cuddy stared at them. He knew he'd never be able to understand them in a hundred years.

The frost crumbled in the warmer air.

The equations narrowed as they were carried on down the wall and across the floor to where the troll had been sitting, until they became just a few expressions that appeared to move and sparkle with a life of their own. This was maths without numbers, pure as lightning.

They narrowed to a point, and at the point was just the very simple symbol: '='.

'Equals what?' said Cuddy. 'Equals what?'

The frost collapsed.

Cuddy went outside. Detritus was now sitting in a puddle of water, surrounded by a crowd of human onlookers.

atrician moved a stack of drawings and sat down.

'These are good,' he said. 'What are they?'

'My cartoons,' said Leonard.

'This is a good one of the little boy with his kite stuck in a tree,' said Lord Vetinari.

'Thank you. May I make you some tea? I'm afraid I don't see many people these days, apart from the man who oils the hinges.'

'I've come to . . .'

The Patrician stopped and prodded at one of the drawings.

'There's a piece of yellow paper stuck to this one,' he said, suspiciously. He pulled at it. It came away from the drawing with a faint sucking noise, and then stuck to his fingers. On the note, in Leonard's crabby backward script, were the words: 'krow ot smees sihT: omeM'.

'Oh, I'm rather pleased with that,' said Leonard. 'I call it my “Handy-note-scribbling-piece-of-paper-with-glue-that-comes-unstuck-when-you-want”.'

The Patrician played with it for a while.

'What's the glue made of?'

'Boiled slugs.'

The Patrician pulled the paper off one hand. It stuck to the other hand.

'Is that what you came to see me about?' said Leonard.

'No. I came to talk to you,' said Lord Vetinari, 'about the gonne.'

'Oh, dear. I'm very sorry.'

'I am afraid it has . . . escaped.'

'My goodness. I thought you said you'd done away with it.'

'I gave it to the Assassins to destroy. After all, they pride themselves on the artistic quality of their work. They should be horrified at the idea of anyone having that sort of power. But the damn fools did not destroy it. They thought they could lock it away. And now they've lost it.'

'They didn't destroy it?'

'Apparently not, the fools.'