title: Python count duplicates over lists in list
body: <p>I have a list containing several other lists. I would like to count how often one element occurs in all of these lists.
  For example:</p>
  
  <pre><code>my_list_of_lists=[[1,2,3,4],[1,2],[],[2,3,5]]
  </code></pre>
  
  <p>the output could be something like:</p>
  
  <pre><code>1: 2
  2: 3
  3: 2
  4: 1
  5: 1
  </code></pre>
  
  <p>A specific element only occurs ones per single list.  </p>
  
  <p>Some piece of code would be very helpful.<br>
  Thank you!</p>