[Boost-users] Any "2D" iterator?

I have on several occasions encountered situations in which it would be advantageous to view a container of containers as a flat sequence. It seems like there ought to be a natural way to do this using an iterator class. For instance, if you have vector v, you ought to be able to design an iterator which begins at v.begin()->begin() and runs until v.end(), over every element of every list. Does such a thing exist in Boost, or is it planned in the near future?