The followings changes to the DUNE-Grid interface I do consider very 
useful and I think these can be achieved during the next year:

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1) Performance issues of the Grid interface: 
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 i) Entity and EntityPointers:
    Entity can be stored (although this might be memory consuming)     
    meaning one can write:

      Entity entity = *it;
 
    For efficiency reasons the Entity class can be implemented as a 
    reference counted pointer which would allow both, easy use and no copying.

    Code of the from

      const Entity& entity = *it ;

    will still be valid.

    The best consequence of this is:

 ii) removal of EntityPointer

    The transition should be not to difficult since the typedef for 
    EntityPointer will point to Entity and on the Entity class we introduce 
    the method

      const Entity* operator -> () const { return this; }

    After this the interface will be much better to understand
    better to use.

    In fact, all methods returning a reference to an object should be revised. 

  iii) For "storage" of Entities in array-like structures we introduce the 
    concept EntityKey (appropriate name to be found) which only 
    contains the minimal information needed to create the corresponding 
    Entity from this. An appropriate method will be available on the Grid class.
    This makes the implementation of Grids and the maintenance of current 
    grids much easier since the we do not have to implement two contrary 
    concepts in one class.

  iv) the method jacobianInverseTransposed (and probably the method 
    jacobian) return not a FieldMatrix but an object specified by the 
    implementation (which can still be a FieldMatrix) which implements all 
    mult methods and casts into a FieldMatrix. With this extension we can 
    implement the jacobianInverseTransposed for Cartesian grids much more 
    efficiently, not the full matvec is needed in this case.

  v) not only the jacobianInverseTransposed but also the Geometry (on Entity and
    Intersection) should not be returned as a reference. This would improve 
    the performance of meta grids which is currently not good, 
    i.e. about 25% are lost when wrapping a grid to provide 
    additional features, e.g. GeometryGrid, PrismGrid, ....


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2) Interface design issues: 
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  i) The boundarySegmentIndex is put into an index set (like any other 
    index) instead of implementing it on an class. This is a contradiction 
    to the existing interface and also something one might only generate 
    when needed.


 ii) Boundary projections have to become "user data". The current 
    approach is a contradiction to separation of data and grid structures. 


 iii) Persistent Index Container: Currently when a DoF mapping is built  
    that should be persistent also in an adaptive computation one would 
    have to use LocalIdSet in some sense which implies O(log(N)) index 
    access instead of O(1). Using AlbertaGrid or ALUGrid together with 
    DUNE-Fem there is a concept to overcome this shortcoming. This should 
    also be introduced in the DUNE-Grid interface and replace the LocalIdSet 
    (not the GlobalIdSet which is still needed as it is). We could 
    introduce a utility class that uses the current IdSet to 
    implement this feature (by default) and which can be specialized 
    for each grid if a better implementation is available. 


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3) Interface extension for special purpose: 
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  i) Easier extension of the Grid interface to introduce new features 
    much more easily and, for example, to allow change of the interface in a better way. 

 ii) The method getRealImp() should be replaced by impl() and should be public which
   would be a good step towards i). Together with this "capabilities" in a namespace
   Extensions are needed that tell whether special features are available or not.