‘I wonder what they’ll be doing in a couple more generations,’ Agata said. ‘Promising women?’

‘You want to wipe us out?’ Medoro joked. ‘Men can’t shed, women can’t be promised. If the words still mean anything, what else can they mean?’

Agata felt a perverse stubbornness rising. ‘Why shouldn’t a mother be able to love her own child? I can’t believe it’s biologically impossible; they just have to find the right pathways. Then everyone would have the choice.’

Medoro was growing less amused. ‘There won’t be an “everyone” if that’s how it ends up.’

‘Of course women can love their children,’ Serena said, trying to conciliate. ‘Women have been aunts, sisters, cousins; no one’s saying we have no feelings. We must have helped raise children all the time. I can love Arianna the way a woman on the home world loves her niece.’

‘That wouldn’t be enough for me,’ Agata said. ‘I don’t want to wipe out men, but if I couldn’t be the one who loved the child the most, I wouldn’t go through with it at all.’

‘That’s just greedy,’ Medoro said. He was keeping his face calm and happy for Arianna, with a voice to match, but Agata could tell that none of this warmth was intended for her.

‘You make your choices, I’ll make mine,’ she said.

‘And what if it’s impossible?’ he taunted her. ‘They can pump as much light into your body as they like, but if the man you want them to wake isn’t in there, he isn’t in there.’

Agata said, ‘Wait and see. Maybe you’ll get a message from Arianna soon enough, letting us all in on the answer.’

‘Spheres are simply connected,’ Lila said. ‘Don’t you think that’s the key?’

‘Perhaps.’ Agata let her rear gaze drift, taking in the crammed bookshelves behind her. Generations of knowledge were packed in there, revelations dating all the way back to Vittorio. She could smell the dye and the old paper – a scent that had always delighted her, promising the thrill of new ideas – but by now she’d absorbed the contents of those shelves so thoroughly that nothing from the past still had the power to astonish her.

‘Any loop on a two-dimensional sphere can be deformed into any other,’ Lila mused, doodling an example on her chest of an elaborate loop being transformed into a simpler one.

‘But on a torus, you can’t change the number of times the loop winds around the space in each dimension, so there are an infinite number of different classes.’ She sketched examples from four of them – pairs of loops that could be transformed into each other, because they shared that distinguishing set of numbers. No amount of stretching or shrinking could take a loop from one class to another.

Lila hesitated, as if expecting Agata to pick up the thread, but after half a lapse she lost patience and prompted her: ‘So what can we say about a four-sphere?’