title: expand data frames inside data frame
body: <p>I have a data frame with the following structure:</p>
  
  <pre><code>   language  subID sessionID                  rdm
        (chr) (fctr)    (fctr)                (chr)
  1     Dutch  13602     13257 &lt;data.frame [676,3]&gt;
  2     Dutch  13602    125354 &lt;data.frame [676,3]&gt;
  3     Dutch  17790     19308 &lt;data.frame [676,3]&gt;
  4     Dutch  37016     38221 &lt;data.frame [676,3]&gt;
  5     Dutch  46830     47890 &lt;data.frame [676,3]&gt;
  </code></pre>
  
  <p>I would like to transform this data frame into a 676*n by 6 data frame - in other words, take the rdm column of data frames and expand it in to the larger data frame.</p>
  
  <p>I would <em>like</em> to be able to do something like:</p>
  
  <pre><code>bind_rows(data_frame$rdm,.id=c(data_frame$sessionID,data_frame$subID,data_frame$language))
  </code></pre>
  
  <p>But, obviously, the ".id" attribute doesn't work that way. How can I get the data frame I'm after?</p>