title: How to sum the value across rows for specific columns?
body: <pre><code>             NYSEARCA:RYE  NYSEARCA:RYE_totalunits  NYSEARCA:PXE  NYSEARCA:PXE_totalunits
  Date                                                                 
  2007-12-31         1              2                           6         3                                  
  2008-01-31         2              3                           6         4   
  2008-02-29         3              4                           3         5   
  2008-03-31         4              5                           3         6   
  </code></pre>
  
  <p>How can i find the total sum of NYSEARCA:RYE_totalunits and NYSEARCA:PXE_totalunits across the rows such that i can achieve this under the new column called total overall units:</p>
  
  <pre><code>             NYSEARCA:RYE  NYSEARCA:RYE_totalunits  NYSEARCA:PXE  NYSEARCA:PXE_totalunits  Total overall units
  Date                                                                 
  2007-12-31         1              2                           6         3                   5                                  
  2008-01-31         2              3                           6         4                   7     
  2008-02-29         3              4                           3         5                   9   
  2008-03-31         4              5                           3         6                   11  
  </code></pre>