Methode aus PLINQ (http://msdn.microsoft.com/en-us/library/ff963547.aspx)
Aggregate<S, A, R>( this ParallelQuery<S> source, Func<A> seedFactory, Func<A, S, A> updateAccumulatorFunc, Func<A, A, A> combineAccumulatorsFunc, Func<A, R> resultSelector);
combine
muß kommutativ sein (http://blogs.msdn.com/b/pfxteam/archive/2008/01/22/7211660.aspx) -- warum?