RST implicit internal links to headers:

```
% pandoc -f rst
Years
-----

Years_
^D
<h1 id="years">Years</h1>
<p><a href="#years">Years</a></p>
```

```
% pandoc -f rst
Years_

Years
-----
^D
<p><a href="#years">Years</a></p>
<h1 id="years">Years</h1>
```

```
% pandoc -f rst
Years and years
---------------

`Years and years`_
^D
<h1 id="years-and-years">Years and years</h1>
<p><a href="#years-and-years">Years and years</a></p>
```

```
% pandoc -f rst
Years and *years*
-----------------

`Years and years`_
^D
<h1 id="years-and-years">Years and <em>years</em></h1>
<p><a href="#years-and-years">Years and years</a></p>
```
