She looked across the cabin at the occulter. Their encounter with the Hurtler and their bomb-removal project offered plausible excuses for all sorts of items ending up in the void, or lost in the dust of Esilio – but she was already afraid that their depleted stores might attract suspicion. Ramiro’s proposed changes would require dozens more proximity sensors – spares that ought to have been packed away neatly in a single large box. Why would they have taken that box out of the storeroom for safekeeping, but then never brought it back?

‘I’m voting with Tarquinia,’ she said. ‘What we’ve got now is physically robust – and we’ll already be hard pressed to build and test the whole swarm before we arrive. This isn’t the time to start making things more complicated.’

For a moment Ramiro looked poised to respond with a further plea, but then he was silent.

Tarquinia said, ‘It’s good to have that settled. Everyone should get some sleep now, and tomorrow we’ll go into production.’

‘Do you want some fresh pictures for your wall?’ Azelio asked, offering Agata a sheaf of papers.

She took them from him. The first drawing showed the mountain coming into view through the window of the Surveyor, with Luisa and Lorenzo standing on the summit waving, very much not to scale. In the second, they’d thrown out a docking rope to the craft and were reeling it in by hand. ‘These are great,’ she said. ‘Thank you.’

Azelio lingered in the doorway. ‘Come in for a while,’ she suggested. He followed her into the cabin. There was only one chair, so she sat on the edge of the desk.

‘I’m going mad,’ Azelio said bluntly. ‘I don’t know what to think any more. I don’t know what to do.’

‘And I don’t know what to tell you.’ Agata had talked him through the situation a dozen times, but he was never satisfied with her account.

‘Tell me that the mountain won’t be destroyed,’ he pleaded. ‘Tell me that everyone will be safe.’

‘The occulters aren’t looking too bad,’ she said. ‘There are benign ways that the disruption might happen; I can promise you that.’

Azelio glanced down at the pile of notes on her desk. ‘And doesn’t everything that could happen, happen? Isn’t that what your diagram calculus says?’

‘No.’ Agata nodded at the pile. ‘For a start, you can only add up diagrams that begin and end in exactly the same way: they all take different paths, but their end points have to be identical. Getting to the disruption with benign sabotage leaves the mountain intact; getting there with a meteor strike hardly brings you to the same state. And even when the end points are identical, all the alternatives you draw for a process just help you find the probability that the process takes place. Those alternatives don’t all get to happen, themselves.’