Interesting! From playing around with it, seems like if the start index is exactly the same as the length, it returns empty array, but if it's further than that it returns nil. That's certainly not something I would have been able to predict, so I'd also be curious if anyone happens to know the explanation for it. My instinct is that it does seem like the type of edge case that might come up with a way to implement it tersely, but that's not a particularly good reason to leak that in the form of user-facing behavior, so hopefully there's a better explanation.

Some additional things I discovered when trying to figure out why it might work like that:

    * the behavior also seems consistent whether using `array.slice(a, b)` or `array[a..b]`
    * `array[array.length]` and `array[array.length + 1]` both return nil