Fix corner case for node collections

[heplesser]

This PR fixes #3213.

[clinssen]

I confirm that this fixes the issues we were seeing in NESTML. Thank you for the quick fix!

[otcathatsya]

Looks good to me, although the test also passes without the fix (on master)

[clinssen]

Looks good to me, although the test also passes without the fix (on master)

That's strange! I double-checked that the test (at least the code snippet from num_n = ... to the assert) fails for me on master and passes on this branch. Do the lines fail for you if you paste them into an ipython session, but the test passes? Or they are both passing?

[otcathatsya]

Looks good to me, although the test also passes without the fix (on master)

That's strange! I double-checked that the test (at least the code snippet from num_n = ... to the assert) fails for me on master and passes on this branch. Do the lines fail for you if you paste them into an ipython session, but the test passes? Or they are both passing?

My bad, forgot to update my fork! One could argue that maybe it should be part of the general slicing test case but that's just nitpicking, lgtm.