title: Merge Two Pandas Dataframes when two columns are list
body: <p>I have two Pandas data frames and they need to be merged.  Example data frames are:</p>
  
  <pre><code>         c1  c2
  pd1 = [[1, [1,2]]
  
           c3  c4
  pd2 = [[1, [1,3]], 
         [2,[2,3]]
  
  result = [[1,1], [1,2]]
  </code></pre>
  
  <p>The join condition is that lists in c2 and c4 have at lease one common element.</p>
  
  <p>I've tried:</p>
  
  <pre><code>result = pd.merge(pd1, pd2, left_on=list('c2'),right_on=list('c4'), how='inner')
  </code></pre>
  
  <p>However, this seems to only join them when the rows in each column are single values like a float, int or string.</p>
  
  <p>I've attacked this problem using nested loops.  This runs like a dog when the sets get large.  Is there a faster way to perform this merge exploiting data frames or is there another way that's better?</p>