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    private isError(data: PositionError | Geoposition): data is PositionError {
        return ((data as PositionError).code !== undefined);
    }

    private $watchPosition(options: GeolocationOptions): Observable<Geoposition> {
        return this.geolocation.watchPosition(options)
            .do((position) => {
                if (this.isError(position)) {
                    throw new PositionError();
                }
            });
    }