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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 vim_janus(uninstall=None):
    '''Install or update Janus, a distribution of addons and mappings for vim.

    More info:
      https://github.com/carlhuda/janus
      Customization: https://github.com/carlhuda/janus/wiki/Customization

    Args:
        uninstall: If not None, Uninstall janus and restore old vim config
    '''
    if uninstall is not None:
        uninstall_janus()
    else:
        if not exists('~/.vim/janus'):
            print_msg('not installed => install')
            install_janus()
        else:
            print_msg('already installed => update')
            update_janus()
        customize_janus()
        show_files_used_by_vim_and_janus()