sl-highlight is a simple language spec for GtkSourceView-based editors, such as Gedit, in order to highlight the syntax of SL language.
In order to install sl-highlight you need to git clone or download the project then go to the cloned/downloaded project folder and do the following in your command line
$ bash install.sh
Then open your editor (gedit, etc)
$ gedit test/fibo.sl
and you should see something like this