@gumblex: This is a good start. It certainly provides a candidate implementation.

First, can I suggest that you remove the changes pertinent to the "at least two elements" and address that in a separate ticket/discussion?

Second, any patch will necessarily need to include a test that fails before the patch and passes after the test.

Third, and least important, the proposed implementation adds a lot of branching and cognitive complexity to an already complicated function. I'd like to work on the implementation to see if we can make it easier to comprehend. That said, practicality beats purity, so if it passes the test, then it's surely acceptable.