I finally looked more closely at your code and it doesn't use views as mine does. The reason I used views was because that was what the book exercise requested. On closer inspection, I'm guessing that your using a vector of "overflow" iterators, where the size of the vector is the number of domains, and each increment of the iterator increments the last element of the vector until it reaches the end, then resets that one and increments the previous overflow iterator in the vector. Is that about right?
That's correct for the "iterative" version. The "recursive" version is modeled after your code. Without the requirement to flatten the view the code was really simple; with this requirement most of the charm is gone and remaining marginal improvements in simplicity certainly don't seem to be worth the loss in performance.
This seems pretty good to me and I don't see how the one I produced could beat that. I may just try it anyway just to see.
I don't think any version that has an effective recursion on the length of the resulting sequence (as opposed to the number of domains) can do very well. On the other hand, iterator triplets and overflow flags look like ugly warts in the realm of functional programming ;) ...Max...