void UpdateNeighbours(IReadOnlyDictionary<CPos, SubCell> footprint)
{
// Include actors inside the footprint too
var cells = Util.ExpandFootprint(footprint.Keys, true);
foreach (var p in cells.SelectMany(c => PreviewsAt(c)))
p.ReplaceInit(new RuntimeNeighbourInit(NeighbouringPreviews(p.Footprint)));
}