title: data.table lag of one column
body: <p>Not sure why this isn't working: </p>
  
  <pre><code>dt = data.table(a = 1:10)
  dt[, c := stats::lag(a)]
  </code></pre>
  
  <p>looks like the new column <code>c</code> is identical to <code>a</code> instead of <code>a</code> lagged. </p>