private static Tuple<Gossip, ImmutableHashSet<UniqueAddress>> Converge(Gossip gossip)
{
var seed = Tuple.Create(gossip, ImmutableHashSet.Create<UniqueAddress>());
return gossip.Members.Aggregate(seed,
(t, m) => Tuple.Create(t.Item1.Seen(m.UniqueAddress), t.Item2.Add(m.UniqueAddress)));
}