title: Prevent unlist to drop NULL values
body: <p>I have a vector of lists and I use <code>unlist</code> on them. Some of the elements in the vectors are <code>NULL</code> and <code>unlist</code> seems to be dropping them.</p>
  
  <p>How can I prevent this?</p>
  
  <p>Here's a simple (non) working example showing this <em>unwanted feature</em> of <code>unlist</code></p>
  
  <pre><code>a = c(list("p1"=2, "p2"=5), 
        list("p1"=3, "p2"=4), 
        list("p1"=NULL, "p2"=NULL), 
        list("p1"=4, "p2"=5))
  unlist(a)
   p1 p2 p1 p2 p1 p2 
   2  5  3  4  4  5 
  </code></pre>