[Draft] Adding Safe indexing to Array

[xwu (Xiaodi Wu)]

@Jens

Isn’t a[checked: i] just as problematic because the notion of checked/unchecked here is confusing?

(-Ounchecked)

As was also mentioned upstreams: people might ask why there is a[checked: i] when a[i] is already checked …

Yeah, and it can also seem like you're saying that you've check i before passing it as an argument. lenient avoids these problems, as does anything that begins if*.