title: How to rbind (concatenate) dataframes stored as element of list of list
body: <p>I have the following list of data frames:</p>
  
  <pre><code>l = list()
  l[[1]] = list(A=iris[1:3,], B=mtcars[1:3,])
  l[[2]] = list(A=iris[5:8,], B=mtcars[5:8,])
  </code></pre>
  
  <p>What I want to do is to concatenate only <code>A</code> element of the list; resulting in:</p>
  
  <pre><code>Sepal.Length Sepal.Width Petal.Length Petal.Width Species
  1          5.1         3.5          1.4         0.2  setosa
  2          4.9         3.0          1.4         0.2  setosa
  3          4.7         3.2          1.3         0.2  setosa
  5          5.0         3.6          1.4         0.2  setosa
  6          5.4         3.9          1.7         0.4  setosa
  7          4.6         3.4          1.4         0.3  setosa
  8          5.0         3.4          1.5         0.2  setosa
  </code></pre>
  
  <p>I tried this but it doesn't give what I want:</p>
  
  <pre><code>&gt; do.call(rbind, l)
       A      B      
  [1,] List,5 List,11
  [2,] List,5 List,11
  </code></pre>
  
  <p>What's the right way to do it?</p>