heapq.merge: after merge the result of merge, the original merge is empty
<p>First I created 2 result of a &amp; b by using merge, but after merge a&amp;b, I found the list of a is empty. </p>

<pre><code>&gt;&gt;&gt; a=merge([1,2],[3,4])
&gt;&gt;&gt; b=merge([4,5],[6,7])
&gt;&gt;&gt; list(a)
[1, 2, 3, 4]
&gt;&gt;&gt; merge(a,b)
&lt;generator object merge at 0x365c370&gt;
&gt;&gt;&gt; list(a)
[]
&gt;&gt;&gt; 
</code></pre>

<p>The final command of list(a) is empty, why merge(a,b) changed a?</p>
