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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_install(**kwargs):
    '''
    Install VirtualBox Guest Additions. Uses the CD, connected by VirtualBox.

    To connect VirtualBox Guest Additions via VirtualBox graphical interface
    press 'Host+D' ('Host' is usually 'Right Ctrl').

    See https://www.virtualbox.org/manual/ch04.html#idp52733088 for more details.

    CLI Example:

    .. code-block:: bash

        salt '*' vbox_guest.additions_install
        salt '*' vbox_guest.additions_install reboot=True
        salt '*' vbox_guest.additions_install upgrade_os=True

    :param reboot: reboot computer to complete installation
    :type reboot: bool
    :param upgrade_os: upgrade OS (to ensure the latests version of kernel and developer tools are installed)
    :type upgrade_os: bool
    :return: version of VirtualBox Guest Additions or string with error
    '''
    with _additions_mounted() as mount_point:
        kernel = __grains__.get('kernel', '')
        if kernel == 'Linux':
            return _additions_install_linux(mount_point, **kwargs)