title: Create a data.frame where a column is a list
body: <p>I know how to add a list column:</p>
  
  <pre><code>&gt; df &lt;- data.frame(a=1:3)
  &gt; df$b &lt;- list(1:1, 1:2, 1:3)
  &gt; df
    a       b
  1 1       1
  2 2    1, 2
  3 3 1, 2, 3
  </code></pre>
  
  <p>This works, but not:</p>
  
  <pre><code>&gt; df &lt;- data.frame(a=1:3, b=list(1:1, 1:2, 1:3))
  Error in data.frame(1L, 1:2, 1:3, check.names = FALSE, stringsAsFactors = TRUE) : 
    arguments imply differing number of rows: 1, 2, 3
  </code></pre>
  
  <p>Why? </p>
  
  <p>Also, is there a way to create <code>df</code> (above) in a single call to <code>data.frame</code>?</p>