p">(</span></span>
<span>    by <span class="op">=</span> <span class="fu">.</span><span class="op">(</span><span class="va">origin</span>, <span class="va">dest</span>, <span class="va">month</span><span class="op">)</span>,</span>
<span>    <span class="fu">at_dt</span><span class="op">(</span><span class="st">"_delay"</span>,<span class="va">summarise_dt</span>,<span class="va">mean</span><span class="op">)</span></span>
<span>           <span class="op">)</span></span></code></pre></div>
<p>Let me expla