This time I'm posting on the right list :) Sorry for the mistake in the last thread.

`iter` has a very cool `sentinel` argument. I suggest an additional argument `sentinel_exception`; when it's supplied, instead of waiting for a sentinel value, we wait for a sentinel exception to be raised, and then the iteration is finished.

This'll be useful to construct things like this: 

    my_iterator = iter(my_deque.popleft, IndexError)

What do you think? 

Ram.