Task <YieldedProvider[]> ExecutePreprocessing(LogSourcePreprocessing prep)
        {
            var ret = prep.Execute();

            if (ret.IsCompleted)
            {
                return(ret);
            }
            items = items.Add(prep);
            changeNotification.Post();
            PreprocessingAdded?.Invoke(this, new LogSourcePreprocessingEventArg(prep));
            return(ret);
        }
        Task <YieldedProvider[]> ExecutePreprocessing(LogSourcePreprocessing prep)
        {
            var ret = prep.Execute();

            if (ret.IsCompleted)
            {
                return(ret);
            }
            items.Add(prep);
            if (PreprocessingAdded != null)
            {
                PreprocessingAdded(this, new LogSourcePreprocessingEventArg(prep));
            }
            return(ret);
        }