Living forever turned out to be much like long-term sex, psychologically tricky; which was why what killed the original colonists was not hardship but boredom. This became the second crisis to hit the worlds.

The first happened no more than a decade after the colonists landed, when the original Chuang Tzu died. No one was watching the Emperor then because these were still early days in the life of the 2023 worlds. He died in the night, peacefully and in his sleep, having told the Librarian that this was what he wanted to happen because he was now very old and very tired.

On fifty-seven worlds, which was the number then inhabited, colonists woke with headaches that got worse as the day went on. By the following week, half the children had nosebleeds or ruptured ears. While tens of thousands panicked, an elderly Indian scientist ran an analysis on the atmosphere, using a semiAI that had been out of date when her grandmother had loaded it onto the ship which brought her family from Calcutta. The answer was surprisingly obvious.

The oxygen-nitrogen mix which the colonists had assumed was natural to all 2023 worlds was thinning, creating elegant day-glow where ultraviolet interacted with oxygen in the upper atmosphere as it leached away into space.

The worlds were dying.

It took a Tibetan monk to solve the problem and that he bothered at all required compromises with his conscience. Historically, at least, the Chuang Tzu represented everything the man hated about Han imperialism and cultural arrogance. All the same, the monk took a small child whose mother had recently died and presented it to the palace, walking right into the Celestial Chamber to leave the child on the throne, like a screaming sack of rubbish.

The palace was empty, the guards gone. The monk was careful not to enquire where... He wasn't afraid of dying, of course. He'd died a hundred times before and could remember most of his lives; at least those of his lives that had happened since he came close to the gates of enlightenment.

Depositing the child, the man explained in simple terms the laws of reincarnation, paying particular attention to the rules governing the appointment of new lamas. He didn't actually tell the silent air around him that emperors came under similar rules or that the ancient Chosen of Heaven had shared such selection procedures with the throne of the Dalai or Panchen Lama, but he might have suggested it.

And he was careful to present reincarnation as real, inasmuch as anything could be real in a quantum universe where facts were both true, false and linked simultaneously.

So now emperors came and went, living out their short reigns in the gaze of those who lived far longer. Maybe this transience was the inspiration for the butterfly cloak or maybe the butterflies had been taken from the mind of the very first emperor, a newly promoted commissar major who'd been nicknamed Chuang Tzu by his grandmother and not as a compliment.

It was hard to know and probably irrelevant, but at some time during the centuries which followed the dreamer's death it became a tradition for each new incarnation to be visited by a butterfly at night.

Something of the desolation and misery of the esclave clung to the walls of Criée Berbere and unnerved those who came in search of bargains from the rug merchants who had taken the slave auctioneer's place.

The buying and selling of people had lasted well into the twentieth century and at one time the going rate in Marrakech was two slaves for a camel, ten for a horse and forty for a civet cat. Those days were gone but there were children in the souks whose grandparents and sometimes even parents had been owned by someone else.

The passage behind Criée Berbere was narrow, high-walled and thick with smoke from a makeshift grill. The height of its walls trapped the grill's thick haze and forced all who used the passage to pass through a cloud of thyme, onion and burning charcoal.

And as the coals over which the skewers of lamb cooked were still a little too hot, the boy behind the grill offered that afternoon's customers cheap paper tissues as protection for their fingers.

He was doing his best not to look at the Englishman.

David Giles sat in a locked doorway, near where the alley turned a corner. His Afghan coat was missing, the Tuarag cross was gone from around his neck and someone had pulled one jeans pocket inside out. He smelt a lot worse than when Moz last saw him but he'd been alive then, that was the difference.

The previous afternoon, Call-me-Dave had been wandering fairly aimlessly from stall to stall in Djemaa el Fna, asking people if they knew somewhere inexpensive he could sleep. Since the second of the day's calls to prayer had barely finished echoing from the square's three minarets this seemed odd to Moz, but the man was a hippie and foreigners were odd by nature.

"What's wrong with your bus?"

It took Dave Giles a second or so to work out that the boy meant his VW Caravette. "The police towed my bus away," he said. "I've got to pay a fine."

"For using the mosque garden?"

Dave Giles shrugged. "They didn't say," he said. "But I need somewhere to stay while my family send money."

Malika's father had offered the man a space on their roof for ten dirham a day, which was nothing for a foreigner, but David Giles turned it down. He was looking for a place with a television. When ould Kasim asked Moz what was wrong with Derb Yassin, he told the old soldier that the foreigner wanted hot water.

It seemed easier.

A dozen heels must have brushed past the nasrani as people slipped between the leather and carpet souks and stepped over his outstretched feet while pretending not to notice he was there.

Glancing round, Moz dropped to a crouch beside the foreigner and slid his good hand quickly inside the man's shirt. He was definitely dead and his wallet on a string was gone. An irregular circle of white around one finger revealed he'd also lost his puzzle ring.

In Call-me-Dave's back pocket, however, Moz found a comic -- Galactic Warrior -- which he pocketed before turning to the boy with the grill, who was dicing onions on an upturned tile.

"How long?" Moz asked.

"How long what?"

"Has the dead man been here."

The boy peered at him from under badly cut hair, his face suspicious. "Which dead man?" he said and went back to his onions.

It took courage for Moz to talk to a policeman, even the kind who wore khaki shirts and carried only small guns. And by the time he found that courage a whole café full of officers knew the small one-armed boy wanted to talk to them.

The café was just outside the gates of the Medina. A place of metal chairs, Formica tables and tiny floor tiles that felt like studs under feet. The walls hid behind sheets of reconstituted local marble better suited for public baths or cheap graves. Café Nouveau had been built in the last decade of French rule in a style that was already out of fashion on the mainland. The police used it because no one else did. Or maybe it was the other way round.

"What do you want?"

The one-armed boy didn't answer the sergeant who spoke. He chose instead an officer with a kinder face, a man younger than the others at that table. So it was only by accident that Moz found himself talking to the most senior police officer present. A graduate who'd taken his degree in Paris. Which might have been enough to cause Aboubakr Abbas endless problems, except for the fact that his uncle had only just retired from the force and he'd spent a childhood hanging around staff canteens in the Hotel de Police in Gueliz.

"What?"

"I've found a body." Moz's words came out so quiet that he repeated them without being asked. "In a passage behind Criée Berbere."