I still use Wintellect's PowerCollections library, even though it is aging and not maintained because it did a good job covering holes left in the standard MS Collections libraries. But LINQ and C# 4.0 are poised to replace PowerCollections...

I was very happy to discover System.Linq.Lookup because it should replace `Wintellect.PowerCollections.MultiDictionary` in my toolkit. But `Lookup` seems to be immutable! Is that true, can you only created a populated Lookup by calling `ToLookup`?