public EvaluatableQbservableProvider(IObservableEvaluator evaluator, Type sourceType)
 {
     Evaluator  = evaluator ?? throw new ArgumentNullException(nameof(evaluator));
     SourceType = sourceType;
 }
 internal EvaluatableQbservable(IObservableEvaluator evaluator)
 {
     Provider   = new EvaluatableQbservableProvider(evaluator, GetType());
     Expression = Expression.Constant(this);
 }