[Znafon]

I think it is consistent, it works a bit like filtering an element from a mathematical set.

Given a set of sheeps, let x be the five-legged sheep is inconsistent because we know neither the existence or uniqueness of shuch sheep, so it raises an exception.

Given a set of sheeps, let x be the subset of five legged sheeps is the empty set because there is no such sheep.

but this may also just be because I internalised Python's behavior.

Some language have a specific value to denote the first thing, for example:

   ["a", "b", "c"][4]

gives `undefined` in JavaScript but it differs from `null` which would be the equivalent to `None` in Python (and I don't think Python has such concept).