title: Pandas and Sets - ValueError: Length of values does not match length of index
body: <p>I am trying to create a new column in my dataframe that contains the intersection of two sets (each contained in two separate columns).  The columns themselves hold sets. </p>
  
  <pre><code>dfc['INTERSECTION'] =  set(dfc.TABS1).intersection(set(dfc.TABS2))
  </code></pre>
  
  <p>I get a Value error.  I was able to do </p>
  
  <pre><code>dfc['LEFT'] = set(dfc.TABS1) - set(dfc.TABS2)
  </code></pre>
  
  <p>no problem. TABS1 and TABS2 have values.</p>
  
  <p>Any thoughts?  Thanks.</p>
  
  <p>I am adding example data below.</p>
  
  <pre><code>GROUP TABS1               TABS2 
  A     {'T1','T2','T3'}   {'T2','T3','T4'} 
  B     {'T5', 'T6'}       {'T6'}
  </code></pre>
  
  <p>Chris gave example, but using very different data set. I am looking for the intersection of TAB1 and TAB2 in a third column 'INTERSECTION.
  As mentioned above, I have no problems with</p>
  
  <pre><code>dfc['LEFT'] = set(dfc.TAB1) - set(dfc.TAB2)
  </code></pre>
  
  <p>This looks like it should be so straight forward... </p>