def single_log(logs):
    """When using @click.argument('log', nargs=-1), this limits the log to zero
    (for stdin) or one log file.

    `logs` -- `list(string)` -- a list of log file names

    `return` -- `string` -- exactly one log file name

    `raise` -- `ValueError` -- if more than one file is passed
    """
    if len(logs) == 0:
        return '-'
    elif len(logs) == 1:
        return logs[0]
    else:
        raise ValueError('Can only process zero or one log files')
