"In theory, if I were casting two spells at once, if I foresaw two conflict elements, I would simply adjust it on the fly. Like directing traffic," she says, although the Ninth House's tiny spaceport could not accommodate more than one shuttle at a time if it had ever needed to, which it had not. "But in two overlapping fields, those moments of interference crop up everywhere, and of course the point of a field is to not require intervention, to create a standing pattern within in an area rather than a specific target."

She lays one diagram over the other and holds them up to the light. "Do you see? Where the patterns would cross and disrupt each other? The heart of this theorem is solve that problem, not just for these two spells, but systematically for any two spells.