c</span> ∥ <span class="hljs-selector-tag">d</span>
∴ <span class="hljs-selector-tag">b</span> ∦ <span class="hljs-selector-tag">c</span>
</code></pre>
<p>可以用下面的语法来表示：</p>
<pre><code>known: <span class="hljs-built_in">parallel</span>(a, b), <span class="hljs-built_in">parallel</span>(c, d), !<span class="hljs-built_in">parallel</span>(a, d)
// 已知 ⇒ 结论
{ <span class="hljs-built_in">parallel</span>(a, b), !<span class="hljs-built_in">parallel</span>(a, d) } ⇒ { !paral