pub fn bfs_arranged<G, N, Tr>(edges: &Arranged<G, Tr>, roots: &Collection<G, N>) -> Collection<G, (N, u32)>
where
    G: Scope<Timestamp=Tr::Time>,
    N: ExchangeData+Hash,
    Tr: for<'a> TraceReader<Key<'a>=&'a N, Val<'a>=&'a N, Diff=isize>+Clone+'static,
{
    // initialize roots as reaching themselves at distance 0
    let nodes = roots.map(|x| (x, 0));

    // repeatedly update minimal distances each node can be reached from each root
    nodes.iterate(|inner| {

        let edges = edges.enter(&inner.scope());
        let nodes = nodes.enter(&inner.scope());

        inner.join_core(&edges, |_k,l,d| Some((d.clone(), l+1)))
             .concat(&nodes)
             .reduce(|_, s, t| t.push((*s[0].0, 1)))
     })
}
Returns pairs (node, dist) indicating distance of each node from a root.
differential-dataflow/src/algorithms/graphs/bfs.rs