 has some missing price values.</span>
<span class='co'># Let's fill them in!</span>
<span class='co'># Note .d=0 tells it to ignore how big the gaps are</span>
<span class='co'># between one period and the next, just look for the most recent insert_date</span>
<span class='co'># .resolve tells it what value to pick if there are multiple</span>
<span class='co'># observed prices for that route/insert_date</span>
<span class='co'># (.resolve is not necessary if .i and .t uniquely identify obs,</span>
<span c