there is some controversy on how to colour the faces of this polyhedron. although the common way to fill in a polygon is to just colour its whole interior, this can result in some filled regions hanging as membranes over empty space. hence, the "neo filling" is sometimes used instead as a more accurate filling. in the neo filling, orientable polyhedra are filled traditionally, but non-orientable polyhedra have their faces filled with the modulo-2 method (only odd-density regions are filled in).