Guards! Guards!

his armpits thoughtfully. He was facing problems of his own.
    He had come here full of rage against book thieves and that rage still burned. But the seditious thought had occurred to him that, although crimes against books were the worst kind of crimes, revenge ought, perhaps, to be postponed.
    It occurred to him that, while of course what humans chose to do to one another was all one to him, there were certain activities that should be curtailed in case the perpetrators got over-confident and started doing things like that to books, too.
    The Librarian stared at his badge again, and gave it a gentle nibble in the optimistic hope that it had become edible. No doubt about it, he had a Duty to the captain.
    The captain had always been kind to him. And the captain had a badge, too.
    Yes.
    There were times when an ape had to do what a man had to do…
    The orangutan threw a complex salute and swung away into the darkness.

    The sun rose higher, rolling through the mists and stale smoke like a lost balloon.
    The rank sat in the shade of a chimney stack, waiting and killing time in their various ways. Nobby was thoughtfully probing the contents of a nostril, Carrot was writing a letter home, and Sergeant Colon was worrying.
    After a while he shifted his weight uneasily and said, “I’ve fought of a problem.”
    “Wassat, Sarge?” said Carrot.
    Sergeant Colon looked wretched. “ Weeell , what if it’s not a million-to-one chance?” he said.
    Nobby stared at him.
    “What d’you mean?” he said.
    “Well, all right , last desperate million-to-one chances always work, right, no problem, but…well, it’s pretty wossname, specific. I mean, isn’t it?”
    “You tell me,” said Nobby.
    “What if it’s just a thousand-to-one chance?” said Colon agonizedly.
    “What?”
    “Anyone ever heard of a thousand-to-one shot coming up?”
    Carrot looked up. “Don’t be daft, Sergeant,” he said. “No one ever saw a thousand-to-one chance come up. The odds against it are—” his lips moved—“millions to one.”
    “Yeah. Millions,” agreed Nobby.
    “So it’d only work if it’s your actual million-to-one chance,” said the sergeant.
    “I suppose that’s right,” said Nobby.
    “So 999,943-to-one, for example—” Colon began.
    Carrot shook his head. “Wouldn’t have a hope. No one ever said, ‘It’s a 999,943-to-one chance but it might just work.’”
    They stared out across the city in the silence of ferocious mental calculation.
    “We could have a real problem here,” said Colon eventually.
    Carrot started to scribble furiously. When questioned, he explained at length about how you found the surface area of a dragon and then tried to estimate the chances of an arrow hitting any one spot.
    “Aimed, mind,” said Sergeant Colon. “I aim .”
    Nobby coughed.
    “In that case it’s got to be a lot less than a million-to-one chance,” said Carrot. “It could be a hundred-to-one. If the dragon’s flying slowly and it’s a big spot, it could be practically a certainty.”
    Colon’s lips shaped themselves around the phrase, It’s a certainty but it might just work . He shook his head. “Nah,” he said.
    “So what we’ve got to do, then,” said Nobby slowly, “is adjust the odds…”

    Now there was a shallow hole in the mortar near the middle bar. It wasn’t much, Vimes knew, but it was a start.
    “You don’t require assistance, by any chance?” said the Patrician.
    “No.”
    “As you wish.”
    The mortar was half-rotted, but the bars had been driven deep into the rock. Under their crusting of rust there was still plenty of iron. It was a long job, but it was something to do and required a blessed absence of thought. They couldn’t take it away from him. It was a good, clean challenge; you knew if you went on chipping away, you’d win through eventually.
    It was the “eventually” that was the problem. Eventually Great A ’Tuin would reach the end of the universe. Eventually the stars would go out. Eventually Nobby might have a bath, although that would probably involve a radical re-thinking of the nature of Time.
    He hacked at the mortar anyway, and then stopped as something small and pale fell down outside, quite slowly.
    “Peanut shell?” he said.
    The Librarian’s face, surrounded by the inner-tube jowls of the Librarian’s head, appeared upside down in the barred opening, and gave him a grin that wasn’t any less terrible for being the wrong way up.
    “Oook?”
    The