title: How to group by all but one columns?
body: <p>How do I tell <code>group_by</code> to group the data by all columns except a given one?</p>
  
  <p>With <code>aggregate</code>, it would be <code>aggregate(x ~ ., ...)</code>.</p>
  
  <p>I tried <code>group_by(data, -x)</code>, but that groups by the negative-of-x (i.e. the same as grouping by x).</p>