A unary relation is a relation of arity 1. Unary relations are the same thing as classes. In this ontology there is no logical distinction between a monadic predicate (unary relation) and a type (class).
(<=> (Unary-Relation ?Relation)
(And (Relation ?Relation)
(Not (Empty ?Relation))
(Forall (?Tuple)
(=> (Member ?Tuple ?Relation) (Single ?Tuple)))))
(Forall (?Tuple) (=> (Member ?Tuple ?Relation) (Single ?Tuple))) (Not (Empty ?Relation)) (Relation ?Relation)
(<= (Arity $X 1) (Unary-Relation $X))
(=> (Unary-Relation ?Relation) (= (Arity ?Relation) 1))
(<=> (Unary-Relation ?Relation)
(And (Relation ?Relation)
(Not (Empty ?Relation))
(Forall (?Tuple)
(=> (Member ?Tuple ?Relation) (Single ?Tuple)))))