observable_subscribe

subscribe(observer?: PartialObserver<T>): Subscription;
subscribe(observerOrNext?: PartialObserver<T> | ((value: T) => void) | null,
            error?: ((error: any) => void) | null,
            complete?: (() => void) | null): Subscription {

    const sink = toSubscriber(observerOrNext, error, complete);

    sink.add(
      this._subscribe(sink) // call fn wicth we pass to constructor
    );
    
    return sink;
  }