(Python 2.7) variable returns "[ ]" instead of desired results
<p>The variable returns "[ ]" instead of desired results.</p>

<pre><code>S = set(['a', 'b','d','z','x','y'])
collect = set()
step = set([''])
while step:
   step = set(a+b for a in step for b in S if len(a+b) == 6)
   collect |= step
print sorted(collect)
</code></pre>

<p>Also how could I make this a loop that plays through the combinations instead of a set?</p>
