def _is_jupytext_file(ntbk):
    """Infer whether a notebook node was created from a Jupytext Markdown file.

    Right now, this just tries to guess based on whether there's a particular piece of
    metadata in the notebook.
    """
    jupytext_meta = ntbk.get('metadata', {}).get('jupytext')
    if jupytext_meta is None:
        return False
    else:
        return jupytext_meta.get('notebook_metadata_filter', '') != "-all"