title: Find the most common pairs in a Dataframe of lists
body: <p>I have a data frame with columns <code>ID, Product</code>. For example, </p>
  
  <pre><code>ID  Product
  1   ['a','b']
  2   ['a','b','e']
  3   ['c','d']
  4   ['a','b','c','d']
  </code></pre>
  
  <p><code>Product</code> is a field of lists where each list contains the products that a person has. For example, a person with <code>ID</code> 1 has the products <code>a</code> and <code>b</code>. I need to find the most popular/common product pairs. In this example, products <code>[a,b]</code> is the most popular. It has to be the most common product pairs since no person can have 1 product.</p>