"Non-negative" is not specified. I don't know exactly what "a, repeated exactly -1 times," looks like, but that's your problem. Closing this issue, because you made it a "your code" issue instead of a kata issue.

Hello, I'm sorry, I weren't on the site for the last two month, since I had a lot of work to do, so I didn't see your message, but I will gladly implement the corrections.

Hi, since this has recently been approved, could you please update it with some corrections:

1. PHPUnit test methods should start with the text test. This is only a problem in the Sample Test Case (see the issue in the kata's discourse).
2. In PHPUnit assertions, the input order is expected first and then actual (user solution), i.e., <assertion>($expected_value, $actual_value).
3. In the Solution Setup, ensure that it is a valid setup; i.e., no error or exception should occur when submitting, even if the user doesn't do anything with it.

You need to think of a more straight-forward way. Or a more complex one. Anyway, a way which does not turn out to be very, very, very ineffective one.

The most straight-forward way I can think of solving it in Haskell leads to Timeout Error