function Subscription(unsubscribe) {
        /**
         * A flag to indicate whether this Subscription has already been unsubscribed.
         * @type {boolean}
         */
        this.closed = false;
        this._parent = null;
        this._parents = null;
        this._subscriptions = null;
        if (unsubscribe) {
            this._unsubscribe = unsubscribe;
        }
    }
@param {function(): void} [unsubscribe] A function describing how to
perform the disposal of resources when the `unsubscribe` method is called.