DataFlow: use [update] at the roots to allow a root to be seeded twice.

src/DataFlow.ml

@@ -200,8 +200,11 @@ module ForCustomMaps
 
   (* Populate the queue with the root variables. *)
 
+  (* Our use of [update] here means that it is permitted for [foreach_root]
+     to seed several properties at a single root. *)
+
   let () =
-    G.foreach_root (fun x p -> V.set x p; schedule x)
+    G.foreach_root update
 
   (* As long as the queue is nonempty, take a variable and examine it. *)