[macintux]

And with a[1:] returning the empty list there’s no way to distinguish between a is empty and a only has one element.

These are, in the end, relatively arbitrary language design decisions.

[akasakahakada]

When you slice a list, you get a list. When you see there is nothing inside the returning list, you know that means end of list, contains zero element. Slicing and indexing return object at different level.

[macintux]

Slicing a list, when the first index is invalid for that list, could easily throw an exception instead.

[zoomablemind]

This should signal an explicit error, which invalid index is indeed. If user believes for some reason the invalid indexing is ok, then it could be caught and handled. No ambiguity.