add function to get root level to abstract assignment

[rkaminsk]

Hi, is this pull request okay with you? If yes, can you merge or give me the okay to merge? Note that I did not find a nice way to deal with submodules. libpotassco has to be merged first and the url of the submodule updated in this request afterwards.

[BenKaufmann]

I merged the PR in libpotassco and I'm fine with this PR. So feel free to update and merge it.

[rkaminsk]

You were commenting on the naming of the function but already merged the request. Are you thinking more about the clingo APIs regarding the naming here?

I went with rootLevel because that is what you used in the solver and I heard it numerous times. Using fixed alone does not seem nice to me because it is hard to make the association to levels from the function signature. Something like fixedLevels might work. Also in the description of isFixed you used top level, so topLevel might also be a candidate.

[BenKaufmann]

You were commenting on the naming of the function but already merged the request. Are you thinking more about the clingo APIs regarding the naming here?

No. I simply misread the comment and thought the function was supposed to return the size of the top-level, i.e. the number of fixed literals but since that is not the case, I'm fine with the name.

Note that there is a difference between topLevel and rootLevel. The former always means level 0 and a literal is fixed iff it is implied on the topLevel. On the other hand, rootLevel is
the "top level" of the current search and is > 0, whenever the search runs "under some assumptions" (i.e. with a non empty guiding path).

[rkaminsk]

I started to think that isFixed also returns true for literals assigned within the root level. I am going to merge this some time later.

Also, in the future I will simply put a branch in the clasp repository instead of using a fork. This makes it easier to work with submodules. I won't merge anything without your okay, though.