 built-in classes) and unrelated</span>
<span class="sd">        ABCs as &#39;virtual subclasses&#39; -- these and their descendants will</span>
<span class="sd">        be considered subclasses of the registering ABC by the built-in</span>
<span class="sd">        issubclass() function, but the registering ABC won&#39;t show up in</span>
<span class="sd">        their MRO (Method Resolution Order) nor will method</span>
<span class="sd">        implementations defined by the registering ABC be callable (no