s` [S]

> All I know for sure is that Czech has retroflexes.
> In Polish it should rather be considered a postalveolar
> (or prepalatal?).
Czech has postalveolars, not retroflexes.
I do transliterate Czech texts with grave accents instead of carons,
but this has no connection with X-SAMPA transcription,
it is simpy because there is no caron in us-ascii nor in iso-8859-1
and the circonflex accent looks too big and ugly in most fonts,
so the grave accent is the best replacement available.

      P. A.