A multi-square in a multi-square doesn't get brushed properly

[TodePond]

It defaults to error red!

[TodePond]

I think the best way to do this is to make the multi-squares "flatten" nested multi-squares. This means we can just avoid any issues around nesting

[Paalui]

I have tried some stuff and generating rules for nested multi-squares seems much easier than for flattened ones.
Flattened multi-squares work for brushes.