title: all group members but the current row
body: <p>I've got a grouped data frame, like so:</p>
  
  <pre><code>df &lt;- data.frame(group = rep(1:4, each=3),
                   lets = rep(LETTERS[1:4], times=3))
  </code></pre>
  
  <p>For each row I would now like to identify all <code>lets</code> within the same group other than the <code>lets</code> of the row itself. Using <code>dplyr</code> I can get <em>all</em> <code>lets</code> thus:</p>
  
  <pre><code>df %&gt;%
    group_by(group) %&gt;%
    mutate(all_lets_in_group = paste(lets, collapse=','))
  </code></pre>
  
  <p>But how do I exclude the <code>lets</code> of the current row from what I feed into <code>paste()</code>?</p>