i have two functions that are inverses of each other (`parse-instant` and format-instant), can i define a relation (instanto string instant) such that: providing string will bind instant to (parse-instant string) and providing instant will bind string to (format-instant instant)

Is this even possible in core.logic?

You can't

(define a relation like that)

You maybe be able to build a constraint out of that(like fd/+)