Add isEmpty so I don't keep fucking up when I assume DSet is a Collection

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@6620 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/main/java/com/threerings/presents/dobj/DSet.java

@@ -152,6 +152,14 @@ public class DSet<E extends DSet.Entry>
     {
     }
 
+    /**
+     * Returns <tt>true</tt> if this set contains no entries.
+     */
+    public boolean isEmpty ()
+    {
+        return _size == 0;
+    }
+
     /**
      * Returns the number of entries in this set.
      */