[Dune] How to get a unique index?

[Andreas Dedner]

 > Hi Patrick!
> That's what the GlobalIdSet is good for.  It has a method id() which
> takes an Entity of arbitrary codim as its argument.  The return type
> is an implementation-dependent type which you can get from the class.
> 
But its not an integer! I just noticed that the documentation is
not correct here - its says an id is a mapping from entity to
integer - but as far as I remember its only a class with
<,== i.e. something one can put into a map.
Was that not the idea?
Andreas