title: Assign values to rows from various dataframes within a list, depending on row number
body: <p>I have a list <code>data.rt</code> of several data.frames, each one with 2 columns and 30 rows. They look something like this</p>
  
  <pre><code>   subject response_time
  1        1          1323
  2        1          1452
  3        1          1566
  4        1          3444
  5        1          9385
  ...      
  </code></pre>
  
  <p>I'd like to add a new column called <code>part</code> to all the dataframes within the list, with 2 factors "A" and "B". It should display "A" for the first 15 rows and "B" for the remaining 15 rows. 
  The result would be something like this for each dataframe</p>
  
  <pre><code>   subject response_time part
  1        1          1323    A
  2        1          1452    A
  3        1          1566    A
  ... 
  15       1          3444    B
  16       1          9385    B 
  17       1          2886    B
  ...      
  </code></pre>
  
  <p>What is the best way to accomplish this?</p>