def _load_fofn(cls, fofn):
        '''Returns dictionary of filename -> short name. Value is None
        whenever short name is not provided'''
        filenames = {}
        f = pyfastaq.utils.open_file_read(fofn)
        for line in f:
            fields = line.rstrip().split()
            if len(fields) == 1:
                filenames[fields[0]] = None
            elif len(fields) == 2:
                filenames[fields[0]] = fields[1]
            else:
                raise Error('Error at the following line of file ' + fofn + '. Expected at most 2 fields.\n' + line)

        pyfastaq.utils.close(f)
        return filenames
Returns dictionary of filename -> short name. Value is None
        whenever short name is not provided