[Gecode] Another abstraction sets...

[Gabor Szokoli]

Christian Schulte wrote:

>I have to admit that I do not subscribe to this reasoning: if there is
>anything that will be extended first by user of Gecode it will be a new
>branching. So providing some functionality for programming distributors
>easily might be a good idea.
>
>  
>
Done.
(Took a while to give up on figuring out superclass- and member element 
construtor execution order and just work around the issue)

If we want to make distributor programming easy, we also need a reversed 
unknown iterator (needs a reversed difference), and a forward and a 
backward "nth element of an interator" operation exposed.
(we have something like the last two already but tied to rangelists 
directly)


Gabor