title: How to use crosstab for multiple columns?
body: <p>I need help using crosstab on the df below.</p>
  
  <pre><code>    a       b       c
  -------------------------
  |   a   |  None |   c   |
  |   a   |   b   |  None |
  |  None |   b   |   c   |
  |   a   |  None |  None |
  |  None |  None |  None |
  </code></pre>
  
  <p>I want to pull rows where more than letter is specified (a&amp;b, a&amp;c, b&amp;c) i.e. rows 1-3. I believe the easiest way to do this is through crosstab (I know I'll get a count but can I also view the rows through this method?). I want to avoid having to write a lengthy 'or' statement to acheive this.</p>
  
  <p>Desired Output:</p>
  
  <pre><code>    a       b       c
  -------------------------
  |   a   |  None |   c   |
  |   a   |   b   |  None |
  |  None |   b   |   c   |
  </code></pre>