in what happens here, especially in
<code>group_dt</code>. First filter by condition
<code>carrier == "AA"</code>, then group by three variables, which are
<code>origin, dest, month</code>. Last, summarise by columns with
“_delay” in the column names and get the mean value of all such
variables(with “_delay” in their column names). This is a very creative
design, utilizing <code>.SD</code> in <em>data.table</em> and upgrade
the <code>group_by</code> function in <em>dplyr</em> (because you never
need