This is a lazily detected absolute filename of the corresponding
        property list file (*.plist). None if it doesn't exist.

def plistfilename(self):
        '''
        This is a lazily detected absolute filename of the corresponding
        property list file (*.plist). None if it doesn't exist.
        '''
        if self._plist_fname is None:
            self._plist_fname = discover_filename(self.label)
        return self._plist_fname