title: R: find the number of common elements of two lists
body: <p>Let's say I have these two lists, and I want to figure out the number of column elements among these two lists:</p>
  
  <pre><code>&gt; xl
  [[1]]
  [1] 1 2 3 4 5
  
  &gt; yl
  [[1]]
  [1] 4 3 5 6 7
  </code></pre>
  
  <p>So, in this example, the answer would be 3. Any suggestions?</p>