title: R's data.table can not find the function "."
body: <p>I read that data.table understands the dot "." as an alias of "list".
  But:</p>
  
  <pre><code>&gt; dt &lt;- data.table(x = c(11, 22), y = c("f", "b"))
  &gt; dt
      x y
  1: 11 f
  2: 22 b
  &gt; dt[,.(y)]
  Error in eval(expr, envir, enclos) : could not find function "."
  &gt;
  </code></pre>
  
  <p>Why is that?</p>