; &gt;                  (Class ?class)
&gt; &gt;                  (forall (?x)
&gt; &gt;                          (=&gt; (instance-of ?x ?subclass)
&gt; &gt;                              (instance-of ?x ?class))))))
&gt; &gt;
&gt; &gt; ;; ISSUE: Given that we are using a modal language, the subclass-of
&gt; &gt; ;; relation is arguably definable: X is a subclass of Y if and only if
&gt; &gt; ;; X and Y are classes and, necessarily, every instance of X is an
&gt; &gt; ;; instance of Y.
&gt; &gt;
&gt; &gt; (d