n p, then it is possible
&gt; &gt;      that z might not occur or that p might not be true of z.
&gt; &gt;  (3) Success or failure.  If an agent x performs an act y whose purpose
&gt; &gt;      is a situation z described by a proposition p, then x is said to be
&gt; &gt;       successful if z occurs and p is true of z. otherwise, x is said to
&gt; &gt;       have failed.  Note that to express these axioms we would need to
&gt; &gt;       introduce lower level classes like "agent" and 'action'.")
&gt; &gt;
&