If an invention has been filed in Hungary, then if within four years from the date of filing of the patent application or within three years from the grant of the patent, whichever period expires last, the patentee has not exploited the invention in the territory of the country to satisfy the domestic demand or if he has not undertaken serious preparations or has not granted a license for such purpose, a compulsory license shall be granted to the applicant for the license, unless the patentee justifies the lack of exploitation.