title: select rows with a change in value
body: <p>I'd appreciate some guidance. I think I should use retain in a data step but I am not too sure how it works yet.</p>
  
  <p>I have a table with 3 columns.</p>
  
  <p>ID, Date, value (numerical).</p>
  
  <p>The table is already sorted by ID1 and Date
  I simply want to select the rows in which the amount changed based on the previous and drop the rows in which it does not. Example below</p>
  
  <pre><code>id      | Date  |amount |
  A       |  01JAN|   1   |
  A       |  02JAN|   1   | &lt;- Drop this row
  A       |  03JAN|   2   |  
  B       |  01JAN|   0   |
  B       |  02JAN|   1   |
  </code></pre>