In fact there are several version of `rst.el` floating around. The
main release location is the Subversion repository. At the moment
Emacs ships the same version AFAICS. You may want to check the log
entries for `rst.el` at

Thanks a lot. I have downloaded the new one and added it in my init file and now I have the function.

Strangely, emacs I use in GNU/Linux (openSUSE) and Windows both have this older rst.el, even though I am using the latest version of emacs available.

Again, thanks for your help!

Best regards,