Permutations of a list of lists
<p>I have a list like this:</p>

<pre><code>l = [['a', 'b', 'c'], ['a', 'b'], ['g', 'h', 'r', 'w']]
</code></pre>

<p>I want to pick an element from each list and combine them to be a string.</p>

<p>For example: 'aag', 'aah', 'aar', 'aaw', 'abg', 'abh' ....</p>

<p>However, the length of the list l and the length of each inner list are all unknown before the program is running. So how can I do want I want?</p>
