pub fn is_same_scc(&self, a: NodeIndex<Ix>, b: NodeIndex<Ix>) -> bool {
        if a == b {
            return true;
        }
        match (self.multi_map.get(&a), self.multi_map.get(&b)) {
            (Some(a_scc), Some(b_scc)) => a_scc == b_scc,
            _ => false,
        }
    }
Returns true if `a` and `b` are in the same scc.
guppy/src/petgraph_support/scc.rs