Function dbsf_multi 

pub fn dbsf_multi<I, L>(lists: &[L], config: FusionConfig) -> Vec<(I, f32)>
where
    I: Clone + Eq + Hash,
    L: AsRef<[(I, f32)]>,

DBSF for 3+ result lists.

Empty Lists

If lists is empty, returns an empty result. Empty lists within the slice contribute zero scores (documents not appearing in those lists receive no z-score contribution from them).

Degenerate Cases

If all scores in a list are equal (zero variance), that list contributes z-score=0.0 for all documents, which is mathematically correct but effectively ignores that list’s contribution.