title: Extract non null elements from a list in R
body: <p>I have a list like this:</p>
  
  <pre><code>    x = list(a = 1:4, b = 3:10, c = NULL)
      x
      #$a
      #[1] 1 2 3 4
      #
      #$b
      #[1]  3  4  5  6  7  8  9 10
      #
      #$c
      #NULL
  </code></pre>
  
  <p>and I want to extract all elements that are not null. How can this be done? Thanks.</p>