Minor Description
Some parts of Grouper are using jexl3 and some are using jexl2. I don't know which functions are using which version.
The last commit for Jexl 2 was December 2011. Jexl 3 was released January 2016. At that time, the documentation page was changed to jexl3 syntax. The syntax for jexl2 is only available in their github repository, or through the internet archive.
Jexl2 syntax is limited, and has bugs that are fixed in jexl3. For example, sets, pragmas, annotations, '??' operator, '?.' and '?[...]' safe-access operators, unicode literals, try/catch, try-with-resources, and functions are not in v2.