title: Referencing a range of columns in dplyr
body: <p>let say I have a data frame <code>df</code> like that</p>
  
  <pre><code>    txt    A1    A2    B1    B2
  1   ala    6      9    12    23
  2   ata    1      3    3     11
  ....
  </code></pre>
  
  <p>I would like to use <code>dplyr</code> for filtering the rows based on a sum of a range of variables. 
  I tried:</p>
  
  <pre><code>filter(df, sum(A2:B1)&gt;10)
  </code></pre>
  
  <p>.... but it does not work.</p>
  
  <p>Could anyone suggest a solution in <code>dplyr</code>? And yes I know it can be done differently by simple subsetting.</p>