😄 I don’t really know but maybe fxxx?

This reads like fail continuation, and if you read Kiselyov's work on logic monad which is used in Kanren, on which core logic is based, you'll find the term pop up. I see it's used in constraint function definition, throwing if the term isn't ground. You'll need to dig into the papers in constraint Kanren if you want to understand why, but I guess it involves checking if the constraint can be checked for the var