public _(GroupByUntil <TSource, TKey, TElement, TDuration> parent, IObserver <IGroupedObservable <TKey, TElement> > observer, IDisposable cancel)
: base(observer, cancel)
{
_parent = parent;
_map = new Map <TKey, ISubject <TElement> >(_parent._comparer);
_nullGate = new object();
}