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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 register(filename):
    '''
    Register a VM

    CLI Example:

    .. code-block:: bash

        salt '*' vboxmanage.register my_vm_filename
    '''
    if not os.path.isfile(filename):
        raise CommandExecutionError(
            'The specified filename ({0}) does not exist.'.format(filename)
        )

    cmd = '{0} registervm {1}'.format(vboxcmd(), filename)
    ret = salt.modules.cmdmod.run_all(cmd)
    if ret['retcode'] == 0:
        return True
    return ret['stderr']