parser.add_argument('-d','--directory',default='',help="if given, this path will be prepended to every entry returned.")
parser.add_argument('-e','--extension',default='',help="if given, this extension will be appended to every entry returned.")
parser.add_argument('-u','--purpose',help="if given, this value will limit the output files to those designed for the given purposes.",choices=('enrol','probe',''))
parser.add_argument('-g','--group',help="if given, this value will limit the output files to those belonging to a particular protocolar group.",choices=('dev','eval','world','optional_world_1','optional_world_2',''))