title: R data.table last row index shortcut
body: <p>In matlab, you can do something like</p>
  
  <pre><code>my_matrix[20:end]
  </code></pre>
  
  <p>which will get the 20th row to the last row in <code>my_matrix</code>. Is there a similar shortcut in <code>R data.table</code>? Something like:</p>
  
  <pre><code>my_matrix[20:end, ]
  </code></pre>