Merged and the buildbot is still green.

Yes. I think the buildbot should be fine. As you said, the iterator didn't
support the full interface before.

Thanks for the review! I've pushed a new version with the changes we discussed
and a merge from default. Should we try merging this version and see what the
buildbots make of it?

We discussed this offline. Just for the record, we discussed that the iterator
does not fully implement the interface of a forward iterator but that wasn't the
case before either. We decided to leave it until it causes a problem, document
it with a comment, and remove another unused increment operator.

Florian, I've created a pull request for you to review:

https://bitbucket.org/malte/downward/pull-requests/6/issue723/diff

Because the main code moved between different files, it is perhaps easiest to
look at the individual commits instead. For the commit where the files were
moved, the only changes in the iterator class were that the references to the
PerStateInformation disappeared. I think it would be fine not to review this
(because the move between files makes the diff annoying).

PerStateInformation currently has const iteration facilities (begin/end), but
they don't actually iterate over the per-state information, but only about the
states of a registry. We want to move this facility to the state registry instead.