One binary relation is the inverse of another if they are equivalent when their arguments are swapped.
(<=> (Holds ?Binary-Relation ?X ?Y) (Holds (Inverse ?Binary-Relation) ?Y ?X)) (=> (Inverse ?Binary-Relation ?Inverse-Relation) (And (Binary-Relation ?Binary-Relation) (= ?Inverse-Relation (Setofall (Listof ?Y ?X) (Holds ?Binary-Relation ?X ?Y)))))