title: finding unique values from a list
body: <p>Suppose you have a list of values</p>
  
  <pre><code>x &lt;- list(a=c(1,2,3), b = c(2,3,4), c=c(4,5,6))
  </code></pre>
  
  <p>I would like to find unique values from all list elements combined. So far, the following code did the trick</p>
  
  <pre><code>unique(unlist(x))
  </code></pre>
  
  <p>Does anyone know a more efficient way? I have a hefty list with a lot of values and would appreciate any speed-up.</p>