Engie Business Rules functions

Hello,

I'm trying to use the functions ISNULL and ISNOTNULL in an expression set but I can't seem to find them anywhere.

Do you know how to access theses functions to be able to verify if a variable is null or not ?

Functions listed in the salesforce documentation here : 

https://help.salesforce.com/s/articleView?id=release-notes.rn_business_rules_engine_intro.htm&release=248&type=5

https://help.salesforce.com/s/articleView?id=sf.operators_and_functions_in_expression_sets.htm&type=5

Thank you