title: Pandas, summing across rows, dependent upon column content
body: <p>I have a weird Dataframe that is set up in the following manner:</p>
  
  <pre><code>header_one | header_two | header_three | header_four | to_sum_one | to_sum_two
       4             5            1              2       header_one   header_three
       2             4            10             12      header_two   header_four
  </code></pre>
  
  <p>Is there a way for me to create a new column by summing the columns indicated the in the sum_one and sum_two fields? For example, the first entry would add up to 5 since we want to sum to_sum_one (4) and to_sum_three (1)</p>
  
  <p>Thank you!</p>