 <code>A</code> implies <code>A</code>.</p>
</blockquote>
<p>But wait, this is a simple although fundamental property of propositional logic: <em>reflexivity of implication</em>.</p>
<p>Let’s take a second example, slightly more complex.</p>
<p>Suppose we have a value <code>v</code> of type <code>A</code> and a function <code>f</code> of type <code>(==&gt; A B)</code>. The question is now:</p>
<blockquote>
<p>How to obtain a value of type <code>B</code>?</p>
</blockquote>
<p>From a computational point of 