Function but_for_cause 

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

But-for causation: effect occurred, and but for cause it would NOT have — i.e. cause is a necessary condition (removing it makes effect unreachable). This is the legal “but-for” / sine-qua-non test.