def _get_contigs_to_use(self, contigs_to_use):
        '''If contigs_to_use is a set, returns that set. If it's None, returns an empty set.
        Otherwise, assumes it's a file name, and gets names from the file'''
        if type(contigs_to_use) == set:
            return contigs_to_use
        elif contigs_to_use is None:
            return set()
        else:
            f = pyfastaq.utils.open_file_read(contigs_to_use)
            contigs_to_use = set([line.rstrip() for line in f])
            pyfastaq.utils.close(f)
            return contigs_to_use
If contigs_to_use is a set, returns that set. If it's None, returns an empty set.
        Otherwise, assumes it's a file name, and gets names from the file