a timepoint just in case the temporal lifespan of the former
&gt; &gt; ;; includes the latter.
&gt; &gt;
&gt; &gt; (forall (?x)
&gt; &gt;         (&lt;=&gt; (instance-of ?x Abstract)
&gt; &gt;              (and (not (exists (?y)
&gt; &gt;                                (or (located-at ?x ?y)
&gt; &gt;                                    (exists-at ?x ?y)))))))
&gt; &gt;
&gt; &gt; (documentation "AXIOM: ?x is physical if and only if it exists at some 
&gt; location at some time.")
&gt; &gt;
&gt; &gt; (forall 