For reference see
https://groups.google.com/d/msgid/pandoc-discuss/cef24253-7731-417c-a1ee-48153c4344bf%40googlegroups.com

```
% pandoc
::: columns
::: column
# A

a
:::
::: column
# B

b
:::
:::
^D
<div class="columns">
<div class="column">
<h1 id="a">A</h1>
<p>a</p>
</div><div class="column">
<h1 id="b">B</h1>
<p>b</p>
</div>
</div>
```
