generate rss channel as xml str

lib/summary.typ

@@ -146,6 +146,13 @@
   return summ
 }
 
+#let summary-sort-key(summ) = -(
+  summ.document-args.date.day()-1 + 31 * (
+    (summ.document-args.date.month()-1) + 
+    12 * summ.document-args.date.year()
+  )
+)
+
 /// Display a list of summary cards.
 ///
 /// min-width is the minimum width of the cards, in px.
@@ -166,12 +173,7 @@
     lang: "raw-css",
   )
   chtml.summary-card-list(attrs: (class: class-name), {
-    for summ in summs.sorted(key: it => 
-      -(it.document-args.date.day()-1 + 31 * (
-        (it.document-args.date.month()-1) + 
-        12 * it.document-args.date.year()
-      ))
-    ) {
+    for summ in summs.sorted(key: summary-sort-key) {
       summ.card
     }
   })