I want to commend you on your testbed. I sort of misunderstood the problem initially, but failure on the test led me to a better understanding. Then some simple-minded would be solutions failed after working for most of the tests.

Well thought out tests are like this: they help finding errors. Most problems proposed in CheckIO come with very weak tests, accepting solutions that are either wrong or terribly bad. So, kudos for you.

Finally, I had to abandon the idea of a simple solution and went in a more complicated direction. In the end, it could be completed as an one-liner of sorts, but that is just tricky coding.