 is that of logical equivalence. We will say that two <code>Logical</code>s <code>x</code> and <code>y</code> are <em>logically equivalent</em> if they have the same truth value. To denote that some expressions <code>p</code> and <code>q</code> of a Logical data type are logically equivalent, we will sometimes also write </p><div class="fragment"><div class="line">p   <span class="keywordflow">if</span> and only <span class="keywordflow">if</span>   q</div>
</div><!-- fragment --><p> which is very common in