evaluate() should always return a boolean

Tried to save us from code duplications. If you like a more simple approach, we can also just add a return value:

    return $this->isNegated() ? FALSE : TRUE;

at the end instead, of course. And keep the existing logic.

Result should be the same.

Yay, code-wise this looks good and clean.
Code looks good, but it's not tested, and i trust no human brain, not even mine, to grok code execution.
We have an install and uninstall test, but no unit test for this simple piece of functionality.
Can you add a test (unit test is enough) or at least test manually?

@geek-merlin totally understand and agree ;) The main reason for this issue was to not forget this. I think it's really minor, but will put this on our list for at least a manual test. But don't expect it too soon. :)

Good, so we'll tacke this further then.