 ```
 % pandoc -f html -t djot
<h1 id="foo" class="a b">Hi</hi>
^D
{#foo .a .b}
# Hi
```

In this one the id is suppressed by the djot writer because
the same one would be automatically generated by the djot reader:
```
% pandoc -f html -t djot
<h2 id="Introduction">Introduction</h2>
^D
## Introduction
```
