it them. The
following codes would not work:</p>
<div class="sourceCode" id="cb9"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="fu"><a href="../reference/mutate.html">mutate_dt</a></span><span class="op">(</span><span class="va">flights</span>,</span>
<span>  gain <span class="op">=</span> <span class="va">arr_delay</span> <span class="op">-</span> <span class="va">dep_delay</span>,</span>
<span>  gain_per_hour <span class="op">=</span> <span class="va">gain</span> <span c