updates to compact representation scheme

Packages/Triangulation_3/TODO

@@ -28,18 +28,13 @@ TDS
 - More compact representation [inspired by a CUJ article for lists] :
   instead of having a cell which stores 4 vertex pointers, it only stores the
   XOR of them.  And a Cell_handle now additionaly stores the 4 vertex pointers
-  of the cell.
-  The tricky part seems to be how to update the correspondance between
-  neighbor(i)/vertex(i) when doing c2 = c->neighbor(i).
-  Maybe it requires storing a permutation table, or storing the vertices
-  in adress order, and squatting bits in the Cell* in order to specify the
-  permutation ?
-  It's definitely worth thinking about it.
-  - Maybe we could remove the requirement of fat handles by storing a little
-    bit more ?  But that seems complicated at first sight.  Let's try
-    to implement the first idea first.
-  - Sharing the neighbor pointers seems less fruitful.
-
+  of the cell, the "context".
+  One problem is the Cell_iterator : it looses the context, so I think that
+  one way to work around this is to write the Cell_iterator as based on the
+  Vertex_iterator (a bit like the reverse used to be made) : we iterate over
+  the vertices, and for each vertex, we compute the incident cells, but we
+  pick only those whose smallest vertex (adress-wise) is the one in question.
+  
 T3
 - degree() devrait-il, au niveau geometrique, ne pas compter le sommet infini ?
 - is_valid devrait verifier l'enveloppe convexe