get all the unique secondary keys in a dictionary python
<p>I have a 2d dictionary eg:</p>

<pre><code> d[a1][b1] = v1
 d[a1][b2] = v2
 d[a2][b1] = v3
 d[a2][b3] = v4
</code></pre>

<p>In the above examples the unique secondary keys are b1, b2, b3?</p>

<p>How do i return the list [b1,b2,b3] in python?
Thanks</p>
