pub fn index(&self, index: usize) -> usize {
        if index < self.zero_prefix {
            0
        }
        else if index - self.zero_prefix < self.smol.len() {
            self.smol[index - self.zero_prefix].try_into().unwrap()
        }
        else {
            self.chonk[index - self.zero_prefix - self.smol.len()].try_into().unwrap()
        }
    }
Like `std::ops::Index`, which we cannot implement as it must return a `&usize`.
differential-dataflow/src/trace/implementations/mod.rs