How to iterate all k-subsets of a range in a specific order?

[Tommi]

Scratch all that. Even if range's 'front' property returned a 
reference, there's no guarantee that it references the same data 
as a copy of that range (created through the 'copy' property). 
So, my D version of that C++ function simply doesn't work... not 
even if 'front' is guaranteed to return a reference. Now this 
seems like a serious defect to me, please tell me I'm wrong.