Fix Ac Unification error and Add new integration test

[MirceaS]

Fixes #2378

---

Review checklist

The author performs the actions on the checklist. The reviewer evaluates the work and checks the boxes as they are completed.

• Summary. Write a summary of the changes. Explain what you did to fix the issue, and why you did it. Present the changes in a logical order. Instead of writing a summary in the pull request, you may push a clean Git history.
• Documentation. Write documentation for new functions. Update documentation for functions that changed, or complete documentation where it is missing.
• Tests. Write unit tests for every change. Write the unit tests that were missing before the changes. Include any examples from the reported issue as integration tests.
• Clean up. The changes are already clean. Clean up anything near the changes that you noticed while working. This does not mean only spatially near the changes, but logically near: any code that interacts with the changes!

[ttuegel]

Are we just removing this check because it is redundant? Or is there another reason?

[MirceaS]

This is the check that I asked you about the other day, the one that Evan removed in his PR that I then had to revert. You suggested I leave it in even if it doesn't solve the Issue linked to this PR.

[ttuegel]

Oh, that is not the check I thought you were removing. It is redundant, and I'm glad to be rid of it, but it doesn't have anything to do with this issue in particular.

[MirceaS]

So, is it fine to leave it in this PR or should I open a new PR?

[ttuegel]

Please update the title of the pull request following these guidelines.

[ttuegel]

Oh, that is not the check I thought you were removing. It is redundant, and I'm glad to be rid of it, but it doesn't have anything to do with this issue in particular.