If a code block is in a list item like:
1. This is a list item with source:
source
Do not use the above, but instead, use:
1. This is a list item with source:
<pre class="fullwidth"><code>
source
</code></pre>
If you want a code block w/ line numbers, it can only be done at the parent level via:
``` { .shell .showlines }
some lines