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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def shutdown(at_time=None):
    '''
    Shutdown a running system

    at_time
        The wait time in minutes before the system will be shutdown.

    CLI Example:

    .. code-block:: bash

        salt '*' system.shutdown 5
    '''
    cmd = ['shutdown', '-h', ('{0}'.format(at_time) if at_time else 'now')]
    ret = __salt__['cmd.run'](cmd, python_shell=False)
    return ret