Appian Function product

Question

I am trying to use the Appian Function product. According to the documentation here is an example: 
 product(10,2) will produce the result 20 
 
 When I try this in my code it throws an error stating 
 
 Not sure why but I put an fn! in front of product and it worked. Took a little while to find this solution. Has anyone else run into this problem? Could this be a problem in my environment or ????

Colton Beck · Answer

You should only use the rule! notation if you're referencing an expression rule you've created in the environment. For OOTB functions, you're correct in using fn! or nothing at all. Since product() is a function, this error is correct as you've called it as a rule that may or may not exist in the environment.