title: How to merge dataframe rows based on empty cells in a column
body: <p>If I have the following dataframe:</p>
  <pre><code>Index Col1 Col2 Col3
  1     10   x    40
  2          y    50
  3          z    60
  4     20   a    30
  </code></pre>
  <p>I would like to merge rows that have a blank Col1 with the previous row that is not blank in Col1.</p>
  <p>Expected output:</p>
  <pre><code>Index Col1 Col2  Col3
  1     10   x,y,z 40,50,60
  4     20   a     30
  </code></pre>
  <p>Is this possible?</p>
  <p>Thanks</p>