anticipate a potential fallback for todos

lib/todos.typ

@@ -27,7 +27,19 @@
     text(size: 2em, weight: "bold", fill:red)[TO-DOs]
     for t in todo-list.final() {
       let l =  label(t.first())
-      list.item(link(l)[TODO n°#t.at(1) p.#locate(l).page() : #t.last()])
+      let locs = query(l)
+      
+      assert(
+        locs.len() >= 1,
+        message: "todo '" + t.first() + "' not found: " +
+          repr(t.last()) + " (query(" + repr(l) + ") = " + repr(locs) + ")"
+      )
+      
+      if locs.len() >= 1 {
+        list.item(link(l)[TODO n°#t.at(1) p.#locs.first().location().page() : #t.last()])
+      } else {
+        list.item(link(l)[TODO n°#t.at(1) p.??? : #t.last()])
+      }
     }
   }
 }