How to use the nbformat.sign.NotebookNotary function in nbformat

"""
        Save notebook model to file system.

        Note: This differs from the NotebookManager.save_notebook in that
        it doesn't have a rename check.
        """
        if not self.is_writable(path):
            raise Exception("Notebook target is not writable")

        bundle_path = self._get_bundle_path(path)
        if not os.path.exists(bundle_path):
            os.mkdir(bundle_path)

        nb = nbformat.from_dict(model['content'])

        notary = sign.NotebookNotary()
        if notary.check_cells(nb):
            notary.sign(nb)

        self.write_notebook(bundle_path, nb)

        if '__files' in model:
            self.write_files(bundle_path, model)

def link_nb(nb_path):
    nb = read_nb(nb_path)
    cells = nb['cells']
    link_markdown_cells(cells, get_imported_modules(cells, Path(nb_path).stem))
    write_nb(nb, nb_path)
    NotebookNotary().sign(read_nb(nb_path))

def _notary_default(self):
        return sign.NotebookNotary(parent=self)

which is always an API-style unicode path,
    and always refers to a directory.

    - unicode, not url-escaped
    - '/'-separated
    - leading and trailing '/' will be stripped
    - if unspecified, path defaults to '',
      indicating the root path.

    """
    
    root_dir = Unicode('/', config=True)

    allow_hidden = Bool(False, config=True, help="Allow access to hidden files")

    notary = Instance(sign.NotebookNotary)
    def _notary_default(self):
        return sign.NotebookNotary(parent=self)

    hide_globs = List(Unicode(), [
            u'__pycache__', '*.pyc', '*.pyo',
            '.DS_Store', '*.so', '*.dylib', '*~',
        ], config=True, help="""
        Glob patterns to hide in file and directory listings.
    """)

    untitled_notebook = Unicode(_("Untitled"), config=True,
        help="The base name used when creating untitled notebooks."
    )

    untitled_file = Unicode("untitled", config=True,
        help="The base name used when creating untitled files."

if spec.varargs not in vars:
                vars[spec.varargs] = []
            vars[spec.varargs].append(value)

    template = template.__name__
    notebook_dir = base_system_path(base, subdir=NOTEBOOKS)
    root = join(notebook_dir, template)
    all_args = all_args if all_args else template
    name = all_args + IPYNB
    path = join(root, name)
    makedirs(root, exist_ok=True)

    log.info(f'Creating {template} at {path} with {vars}')
    notebook = load_notebook(template, vars)
    # https://testnb.readthedocs.io/en/latest/security.html
    NotebookNotary().sign(notebook)
    if exists(path):
        log.debug(f'Deleting old version of {path}')
        unlink(path)
    with open(path, 'w') as out:
        log.info(f'Writing {template} to {path}')
        nb.write(notebook, out)
    return join(template, name)

else:
            if spec.varargs not in vars:
                vars[spec.varargs] = []
            vars[spec.varargs].append(value)

    template = template.__name__
    base = join(notebook_dir, template)
    all_args = all_args if all_args else template
    name = all_args + IPYNB
    path = join(base, name)
    makedirs(base, exist_ok=True)

    log.info(f'Creating {template} at {path} with {vars}')
    notebook = load_notebook(template, vars)
    # https://testnb.readthedocs.io/en/latest/security.html
    NotebookNotary().sign(notebook)
    if exists(path):
        log.warning(f'Deleting old version of {path}')
        unlink(path)
    with open(path, 'w') as out:
        log.info(f'Writing {template} to {path}')
        nb.write(notebook, out)
    return join(template, name)

def update_nb_metadata(nb_path=None, title=None, summary=None, keywords='fastai', overwrite=True, **kwargs):
    "Creates jekyll metadata for given notebook path."
    nb = read_nb(nb_path)
    data = {'title': title, 'summary': summary, 'keywords': keywords, **kwargs}
    data = {k:v for (k,v) in data.items() if v is not None} # remove none values
    if not data: return
    nb['metadata']['jekyll'] = data
    write_nb(nb, nb_path)
    NotebookNotary().sign(nb)