d <code>B</code> is true.</li>
</ul>
</blockquote>
<p>In what is called <em>natural deduction</em> in logic this is exactly the definition of the <em>introduction rule for conjunction</em>.</p>
<p>Let’s now take the <code>fst</code> function, whose type becomes:</p>
<pre><code class="language-clojure">(==&gt; (and-by-all-means A B)
     A)
</code></pre>
<p>So:</p>
<blockquote>
<p>If we know that the conjunction of <code>A</code> and <code>B</code> is true Then <code>A</code> “alone” is true</p>
</bloc