howto produce a myst/markdown format

Hi, I’d like to give the myst markdown variant a try

https://jupytext.readthedocs.io/en/latest/formats.html?highlight=myst#myst-markdown

but have not been able to find the right syntax, here’s what I tried

$ jupytext --set-formats markdown:myst  my-notebook.md
[jupytext] Reading w9-s2-c1-decorateurs.md
[jupytext] Updating notebook metadata with '{"jupytext": {"formats": "markdown:myst"}}'
<stack-trace>
jupytext.formats.JupytextFormatError: myst is not a valid format name. Please choose one of light, pandoc, spin, hydrogen, markdown, nomarker, sphinx, rmarkdown, percent

as a side question, I have just obtained this my-notebook.md by running

$ jupytext --to markdown my-notebook.ipynb

and am a little confused as to which specific markdown variant is being used; the .md file reads like this

---
jupyter:
  jupytext:
    formats: md
    text_representation:
      extension: .md
      format_name: markdown

thanks again for the awesome tool !

[parmentelat]

ah <mental slap on the forehead> ! stupid me then 😃

[mwouts]

Closing as #584 was merged - the error message will be much more informative now.