Concepts
========

- Assignable

  Has copy constructor and assignment operator

- DefaultConstructible

  has constructor without arguments

- EqualityComparable

  has operator== and operator!=

- LessThanComparable

  has operator<

- Dereferenceable

  has operator* and operator->
  (it is not required that these operations yield a well defined
  value)

- MutableIterator

  non-const methods on dereferenced value may be called

- ImmutableIterator

  only const methods on dereferenced value may be called


Application to our specific classes
===================================

Class           Assignable DefaultCons Equality LessThan Immutable

Grid              no         no          no(?)    no

Entity            no         no          no       no

GeometryType      yes        yes         yes      yes

Geometry          no         no          no       no

EntityPointer     yes        no          yes      no       yes

LevelIterator     yes        no          yes      no       yes

LeafIterator      yes        no          yes      no       yes

HierIterator      yes        no          yes      no       yes

IntersIterator    yes        no          yes      no  (not dereferenceable!)

IndexSet          no         no          no       no

IdSet             no         no          no       no




Questions:


- If a feature is no, should the corresponding method be explicitely
  forbidden, i.e. the method is private?

- Do we want EntityPointer, LevelIterator, LeafIterator, HierIterator
  assignable? Is this (in principle) possible with our
  implementations? (Robert: not for 1.0, but in principle).

- An Equality test on grid objects might be useful. Should we include
  this? (In principle, not now).

Notes:

- If a type is neither Assignable nor DefaultConstructible this means
  that a *user* cannot create any object of this class. Of course this
  was our intention. Only the grid is authorized to create (iterators)
  to such objects.