Function is_overdetermined 

pub fn is_overdetermined(
    edges: &[NQuin],
    roots: &[u64],
    causes: &[u64],
    effect: u64,
) -> bool

Causal overdetermination (joint liability): effect occurred, there are ≥2 candidate causes, and no single one is but-for — removing any one alone still yields the effect (another sufficient cause remains). Liability is then shared across all of them.