public static IPInvokeObservable <T> Distinct <T, TKey>(this IPInvokeObservable <T> @this, Func <T, TKey> func)
 => @this.Distinct(func, EqualityComparer <TKey> .Default);
 public static IPInvokeObservable <T> Distinct <T>(this IPInvokeObservable <T> @this)
 => @this.Distinct(EqualityComparer <T> .Default);