pub fn index_span(&self, index: usize) -> Span {
        let mut best = None;

        for (i, span) in self.spans.iter() {
            if i > &index {
                break;
            }
            best = Some(span);
        }

        return best.unwrap().clone();
    }
Look up the nearest span at or before the index of a specific bytecode
op.
passerine-common/src/lambda.rs