title: summarizing 2 columns into one based on a third index column
body: <p>I have the following dataframe</p>
  
  <blockquote>
  <pre><code>  first_char second_char type
  1          a           b  1/1
  2          a           b  0/1
  3          a           b  0/1
  4          c           d  1/1
  5          c           d  0/1
  6          c           d  0/0
  </code></pre>
  </blockquote>
  
  <p>I would like to combine these columns into one as such:</p>
  
  <pre><code>1       bb
  2       ab
  3       ab
  4       dd
  5       cd
  6       cc
  </code></pre>
  
  <p>The <code>type</code> column contains the indeces separated by a forward slash for first_char and second_char columns.</p>