He did this for amusement and because he knew that it worried the Library. Sometime soon, Zaq would have to face the glass and listen as the Librarian explained what Zaq already knew. That a young assassin, crazed with cold and loneliness, was working her way across the bridge between plateaux. A dark-haired, thin child who held conversations with the air and carried a large knife with which to rip out the heart of the Chuang Tzu.

Everyone Zaq met in the palace thought he was hiding from the danger facing him. Zaq could see it in their faces and hear it in the way conversations stilled as he swept through the corridors.

They were wrong.

Zaq knew all about the cold assassin and he assumed the Library knew about the prisoner trapped on a sun-baked island who carried the emperor's dreams. War could be a very complex business and weapons were not always what they seemed.

"Majesty..."

"Excellency," Zaq corrected. The soldier should be able to work out for himself that he stood in front of a duke and not an emperor. Zaq tried hard to recall the man's name, screwing up his face as he did so.

Tso Chi?

Li Han?

He could ask the Library, only then Zaq would have to talk to the Librarian about the other thing and that was exactly what he was trying to avoid.

"General Ch'ao Kai," said Chuang Tzu, and saw surprise turn to pleasure as the bannerman understood he'd been recognized. "This must be important." They both understood the hidden rebuke. All were forbidden to acknowledge the Chuang Tzu's existence and there were no exceptions.

The old soldier nodded. "I beg Your Excellency's permission to deploy troops outside the city wall."

"And why would you want to do that?" Zaq asked without thinking. He should have said something like Deploy troops, for what reason? But more and more these days he forgot to keep his thoughts formal, his face measured.

"Just manoeuvres, Excellency. The troops need exercise. I thought you might approve of the idea."

The man lied badly.

Zaq smiled. It was a gentle smile, the kind one might expect from either a poet faced with a particularly beautiful waterfall or a scholar presented with a scroll no other scholar had seen for a thousand years. The kind an emperor might give in the face of death.

"No," he said. "I don't think so."

Real anguish crossed the old General's face. So convincing in fact that Zaq was impressed yet again with the sheer inventiveness of the Library.

"They can exercise in front of the Taihe Dien," Zaq said, the nearest he was prepared to get to a compromise.

"And in the outer city, Excellency?"

"The square," Zaq said firmly. "Then I can watch them from the Supreme Harmony Gate." He wouldn't, of course. In all his years as Chuang Tzu he'd only ever watched the troops on one occasion. He understood the levels of skill required, but had little personal interest in the use of weapons.

Smiling at the old man, Chuang Tzu touched him lightly on his shoulder and turned to go, leaving General Ch'ao Kai looking after him. Somehow his generals were always old, always bearded and dressed in elaborate armour that seemed to consist mostly of polished tortoiseshell and red ribbon. Red was the colour of luck and given the amount used in the Forbidden City, Zaq should have been very lucky indeed.

"Majesty..." The voice came down the corridor behind him and the fact it was aimed at his back was such a breach of court etiquette that Ch'ao Kai had to be truly desperate. Zaq could stop or he could keep walking and send a clear sign that he did not choose to hear what the General was so desperate to say.

He stopped, that was all, stopped and stood in silence, dressed in a ceremonial chao pao as if conducting negotiations or taking a wife. And all those not watching Tris struggle her way across the bridge listened to a grey-haired, sad-eyed man tell his Emperor that the Librarian urgently requested a word.

And those fifteen billion watched Chuang Tzu shake his head, surprisingly regretfully, and then keep walking.

Many of them were still watching when Zaq skirted the Western Palaces and the Thousand Autumns Pavilion on his way to the rockeries and walkways of the Yuhua Yuan, the Butterfly Garden.

They didn't realize it yet but the Emperor had made a decision. Here was where he intended to stay. Not just for the morning or the rest of that day. Zaq would stay for however many nights and days it took for the Library to bring him the assassin.

And when this happened and the stranger had made it across the bridge and into the Forbidden City, Zaq would stare the young assassin in the eyes and ask the question 148 billion people wanted answered.

Why?

And then, if he was lucky, Zaq might finally be allowed to sleep.

"Shit," Luca said, then apologized.

Tris smiled. She could have told him words far worse. Some so vile he'd probably need coaching in their meaning. This thought kept her amused until he swore again, which was soon.

"What?" she said, stepping from one plank to another.

"Just this," said Luca. So thick was the falling snow that there were whole hours, sometimes longer, in which Luca drifted from sight behind her and Tris was anchored to his absence through a haze of floating white that stung like memory as it turned to tears on her face.

She had frostbite, her lips were frozen into a rictus grin and her ears felt missing, along with most of her fingers and both her feet from the ankles down. Only the wind was in their favour, having switched direction and turned to a light breeze that no longer threw snow directly into their faces.

"Keep going," Luca said.

Tris's world was reduced to narrow and uncertain strips of ice which glazed the ancient wooden planks over which she stepped. Luca's blade was stuck through her belt and she knew this was a stupid way to carry a naked weapon, but having the blade visible reminded her why she put one step after another instead of just doing what she wanted to do, which was curl up into a ball and give herself back to sleep. Instead she put one foot in front of the other and kept walking...

"Why have you stopped?"

Tris turned to find Luca at her side and realized she'd been staring over the edge of the bridge without even realizing it, both hands gripping one of the main cables. And she'd been standing there for so long that snow had made gloves of the backs of her fingers.

"I can't remember," Tris said.

"You okay?"

"Sure," said Tris, then thought about it. "I don't even know where I am," she said, gesturing wearily at the way ahead and then turning to include what little could be seen of the bridge behind, which was a half-dozen frosted planks that did scant justice to the hours they'd been walking. "Of course I'm not okay," she said. "How could I be?"

"We're almost there." Luca put one hand lightly on her shoulder and appeared not to notice when Tris shook it off.

"Two days at the most," she said. "Wasn't that what you told me?" They both knew it was and yet Luca seemed unfazed by the endlessness of the planks and almost happy that the ice glazing each one was becoming thinner by the hour. "So what changed?"

"I think," said Luca, "it's more a case of what's changing." He glanced at the cloud and then at the snowflakes which continued to fall long after they passed where Luca stood on the bridge beside Tris, both of them gripping a fat cable and staring into the abyss.

"‘What's changing’?"

"Well," Luca said, "there aren't that many alternatives. And since I doubt that time is expanding it must be the bridge."

"How can a bridge expand?"

"How can it not?" Luca slipped off his cloak and did something with his hands that unravelled whole layers of material not visible a moment earlier. "You sleep now," he said. "I'll keep guard."