title: Flatten list column in data frame with ID column
body: <p>My data frame contains the output of a survey with a select multiple question type. Some cells have multiple values.</p>
  
  <pre><code>df &lt;- data.frame(a=1:3,b=I(list(1,1:2,1:3)))
  df
    a       b
  1 1       1
  2 2    1, 2
  3 3 1, 2, 3
  </code></pre>
  
  <p>I would like to flatten out the list to obtain the following output:</p>
  
  <pre><code>df
    a       b
  1 1       1
  2 2       1
  3 2       2
  4 3       1
  5 3       2
  6 3       3
  </code></pre>
  
  <p>should be easy but somehow I can't find the search terms. thanks.</p>