Function runtime_index_sequence 

pub fn runtime_index_sequence<S>(
    cx: &mut Cx,
    source: Arc<S>,
    first_index: i64,
) -> Result<Value>
where
    S: RuntimeIndexSource + 'static,

Builds a lazy sequence over contiguous integer keys starting at first_index.

The sequence stops at the first missing key. This is intentionally a generic array projection, not a language-specific length or border rule.