Function is_necessary_cause 

pub fn is_necessary_cause(
    graph: &[NQuin],
    root: u64,
    candidate: u64,
    effect: u64,
) -> bool

But-for causal necessity: candidate is a NECESSARY cause of effect (from origin root) iff effect is reachable from root, but is NOT reachable once candidate is removed from the causal graph. The attribution/liability test (“would the harm have occurred but for this agent’s act?”). Zero-heap.