Oh sorry, this is setting a layers property on the _Facet class.  This is because Facets are built on the base mesh, not the extruded mesh.  And then when we build an ExtrudedMesh we make the facets behave correctly by pointing them at the base mesh facets and setting a layers property.

I would be much happier if instead ExtrudedMesh gained the interior and exterior facet properties directly and that in the constructor you build new Facets objects and pass a layers constructor there.  (If someone accidentally uses a base mesh that they've subsequently extruded with this change, they would get bad results doing facet integrals).