How to use the fbs.builtin_commands._util.require_frozen_app function in fbs

def installer():
    """
    Create an installer for your app
    """
    require_frozen_app()
    linux_distribution_not_supported_msg = \
        "Your Linux distribution is not supported, sorry. " \
        "You can run `fbs buildvm` followed by `fbs runvm` to start a Docker " \
        "VM of a supported distribution."
    try:
        installer_fname = SETTINGS['installer']
    except KeyError:
        if is_linux():
            raise FbsError(linux_distribution_not_supported_msg)
        raise
    out_file = join('target', installer_fname)
    msg_parts = ['Created %s.' % out_file]
    if is_windows():
        from fbs.installer.windows import create_installer_windows
        create_installer_windows()
    elif is_mac():

def sign():
    """
    Sign your app, so the user's OS trusts it
    """
    require_frozen_app()
    if is_windows():
        from fbs.sign.windows import sign_windows
        sign_windows()
        _LOG.info(
            'Signed all binary files in %s and its subdirectories.',
            relpath(path('${freeze_dir}'), path('.'))
        )
    elif is_mac():
        _LOG.info('fbs does not yet implement `sign` on macOS.')
    else:
        _LOG.info('This platform does not support signing frozen apps.')