title: Unnesting a list of lists in a data frame column
body: <p>To unnest a data frame I can use:</p>
  
  <pre><code>df &lt;- data_frame(
      x = 1,
      y = list(a = 1, b = 2)
  )
  
  tidyr::unnest(df)
  </code></pre>
  
  <p>But how can I unnest a list inside of a list inside of a data frame column?</p>
  
  <pre><code>df &lt;- data_frame(
      x = 1,
      y = list(list(a = 1, b = 2))
  )
  tidyr::unnest(df)
  </code></pre>
  
  <p>Error:</p>
  
  <blockquote>
    <p>Each column must either be a list of vectors or a list of data frames [y]</p>
  </blockquote>