python operations with sets
<p>I use python only occasionally, sorry for a seemingly trivial question</p>

<pre><code>&gt;&gt;&gt; a = set(((1,1),(1,6),(6,1),(6,6)))
&gt;&gt;&gt; a
set([(6, 1), (1, 6), (1, 1), (6, 6)])
&gt;&gt;&gt; a - set(((1,1)))
set([(6, 1), (1, 6), (1, 1), (6, 6)])
&gt;&gt;&gt; a.remove((1,1))
&gt;&gt;&gt; a
set([(6, 1), (1, 6), (6, 6)])
</code></pre>

<p>why '<code>-</code>' operator didn't delete the element but the <code>remove</code> did ?</p>
