An instance i CAN-BE-ONE-OF a set of classes S iff i is an instance of at most one of the classes. Inside the definition of a class, the form (CAN-BE-ONE-OF ?i (setof C1 C2 ...)) is a convention for stating (subclass-partition class (setof C1 C2 ...)). The two forms are equivalent if each class C1, C2, ... is also defined to be a subclass of C.
(<=> (Can-Be-One-Of ?Instance ?Set-Of-Classes) (And (Forall (?Class) (=> (Member ?Class ?Set-Of-Classes) (Class ?Class))) (Forall (?Class) (=> (Member ?Class ?Set-Of-Classes) (Instance-Of ?Instance ?Class) (Forall (?Other-Class) (=> (Member ?Other-Class ?Set-Of-Classes) (Not (= ?Other-Class ?Class)) (Not (Instance-Of ?Instance ?Other-Class))))))))