title: summarise_at dplyr multiple columns
body: <p>I am trying to apply a complex function on multiple columns after applying a group on it.</p>
  
  <p>Code example is:</p>
  
  <pre><code>library(dplyr)
  data(iris)
  
  add = function(x,y) {
      z = x+y
      return(mean(z))
  }
  
  iris %&gt;%
    group_by(Species) %&gt;%
    summarise_at(.vars=c('Sepal.Length', 'Sepal.Width'), 
             .funs =  add('Sepal.Length', 'Sepal.Width' ) )
  </code></pre>
  
  <p>I was expecting that the function would be applied to each group and returned as a new column but I get:</p>
  
  <pre><code>Error in x + y : non-numeric argument to binary operator
  </code></pre>
  
  <p>How can I get this work?</p>
  
  <p>Note my real problem has a much more complicated function than the simple <code>add</code> function I've written here that requires the two columns be fed in as separate entities I can't just sum them first. </p>
  
  <p>Thanks</p>