Split words in a nested list into letters
<p>I was wondering how I can split words in a nested list into their individual letters such that</p>

<pre><code>[['ANTT'],['XSOB']]
</code></pre>

<p>becomes</p>

<pre><code>[['A', 'N', 'T', 'T'], ['X', 'S', 'O', 'B']]
</code></pre>
