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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 additions_umount(mount_point):
    '''
    Unmount VirtualBox Guest Additions CD from the temp directory.

    CLI Example:

    .. code-block:: bash

        salt '*' vbox_guest.additions_umount

    :param mount_point: directory VirtualBox Guest Additions is mounted to
    :return: True or an string with error
    '''
    ret = __salt__['mount.umount'](mount_point)
    if ret:
        os.rmdir(mount_point)
    return ret