Editing Arrangement of hyperplanes (section)

==Technicalities==

Sometimes it is convenient to allow the '''degenerate hyperplane''', which is the whole space ''S'', to belong to an arrangement.  If ''A'' contains the degenerate hyperplane, then it has no regions because the complement is empty.  However, it still has flats, an intersection semilattice, and faces.  The preceding discussion assumes the degenerate hyperplane is not in the arrangement.

Sometimes one wants to allow repeated hyperplanes in the arrangement.  We did not consider this possibility in the preceding discussion, but it makes no material difference.