<p>Does anybody know if there is a built-in function in Mathematica for getting the lhs of downvalue rules (without any holding)?  I know how to write the code to do it, but it seems basic enough for a built-in</p>

<p>For example:</p>

<pre><code>a[1]=2;
a[2]=3;
</code></pre>

<p><code>BuiltInIDoNotKnowOf[a]</code> returns <code>{1,2}</code></p>

This seems to work; not sure how useful it is, though:
```
```
a[1] = 2
a[2] = 3
a[3] = 5
a[6] = 8
Part[DownValues[a], All, 1, 1, 1]
```
```