Considering the valid indexes are extended by "-1" for backwards
iterators and "size()" for forward iterators, it's obvious that a
not-yet-at-the-end-in-the-other-direction iterator can be incremented
once, but not necessarily twice.

Why this code even wants to increment it twice isn't obvious to me.

Change-Id: I578c8c6202049ebe6dbed41b8276a6bfa0566bbc