public ImmutableHashSet<UniqueAddress> AllUnreachableFrom(UniqueAddress observer)
{
var observerRows = ObserverRows(observer);
if (observerRows == null) return ImmutableHashSet.Create<UniqueAddress>();
return
ImmutableHashSet.CreateRange(
observerRows.Where(p => p.Value.Status == ReachabilityStatus.Unreachable).Select(p => p.Key));
}