Does a Grid Implementation Have to Provide Entities of all Codimensions?
The DUNE grid interface contains methods for access to grid entities of all codimensions. However, for some entities such as faces and edges in a 3d grid, this is sometimes difficult to implement. Also, most PDE codes will not need them anyways. Therefore, currently grid implementations are only required to provide vertices and elements. All others are optional. Please refer to the documentation for the individual grids to see what codimensions they implement. Lagrangian finite elements of order higher than one have degrees of freedom on edges and faces. In order to implement them you don’t need the actual entities, just getting their indices is enough. Therefore, the method subIndex
