title: Find summary statistics for a python dictionary with multiple values
body: <p>suppose I have a dictionary:</p>
  
  <pre><code>a_dic = {'file1':["a","b","c"],
      'file2':["b","c","d"],
      'file3':["c","d","e"]}
  </code></pre>
  
  <p>I want to write a function to be able to return a dictionary/dataframe to find the occurrence of the keys like:</p>
  
  <pre><code>occurrence = {'a':1, 'b':2, 'c':3, 'd':2,'e':1}
  </code></pre>