public static bool Completed <T>(
this ITestableObserver <T> o) => o.GetValues(NotificationKind.OnCompleted)
.Select(_ => true)
.SingleOrDefault();
public static IEnumerable <T> GetValues <T>(
this ITestableObserver <T> o) => o.GetValues(NotificationKind.OnNext)
.Select(x => x.Value);