Issue #139: Unexpected behavior of plus operator

[pkluegl]

What's in the PR

• fixed rule inference
• some cleanup

How to test manually

• ...

Automatic testing

• [x] PR adds/updates unit tests

Documentation

• [ ] PR adds/updates documentation

Organizational

• [ ] PR includes new dependencies. _{^{Only dependencies under approved licenses are allowed. LICENSE and NOTICE files in the respective modules where dependencies have been added as well as in the project root have been updated.}}