Let's have Set implement forEach, like Map does.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@5500 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/as/com/threerings/util/ExpiringSet.as

@@ -62,6 +62,14 @@ public class ExpiringSet extends EventDispatcher
         _data.length = 0;
     }
 
+    // from Set
+    public function forEach (fn :Function) :void
+    {
+        for each (var e :ExpiringElement in _data) {
+            fn(e);
+        }
+    }
+
     // from Set
     public function size () :int
     {