title: Union in Sage/Python
body: <p>I have a list of lists (that might have any number of lists as entries) and want to find the union of this list (as a set) in Sage. I would think there is a short existing command already but I was not able to find it.</p>
  <p>Here an example:</p>
  <pre><code>W=[[2, 1], [2], [3, 1], [1]]
  T=W.union()
  display(W)
  display(T)
  </code></pre>
  <p>The desired output is T=[1,2,3] but it seems the command &quot;union&quot; does not work.</p>