title: Creating a column to enumerate a set of events
body: <p>I have data on a set of events that happen in some order.  I would like a column that enumerates that set of events: this row is the first time, this row is the second time etc.  The data will look like the following:</p>
  
  <pre><code>S   Time
  A     3
  A     4
  A     5
  A     10
  B     4
  B     9
  B     1
  B     37
  </code></pre>
  
  <p>Where S is some kind of session ID and time is obviously time.  I would want the following result column added:</p>
  
  <pre><code>S   Time   Order
  A     3      1
  A     5      3
  A     4      2
  A     10     4
  B     4      2
  B     9      3
  B     1      1
  B     37     4
  </code></pre>
  
  <p>For each of the session IDs, I want to have a column that gives an ordering on the rows based on time.  I'm using <code>dplyr</code> and I'm hoping for a concise dplyr way of doing this.</p>