Index: src/java/org/apache/lucene/search/DocIdSet.java
===================================================================
--- src/java/org/apache/lucene/search/DocIdSet.java	(revision 738168)
+++ src/java/org/apache/lucene/search/DocIdSet.java	(working copy)
@@ -18,11 +18,17 @@
  */
 
 import java.io.IOException;
+import org.apache.lucene.util.SortedVIntList;
 
 /**
  * A DocIdSet contains a set of doc ids. Implementing classes must provide
  * a {@link DocIdSetIterator} to access the set. 
  */
 public abstract class DocIdSet {
-	public abstract DocIdSetIterator iterator() throws IOException;
+
+  /** An empty {@code DocIdSet} instance for easy use (this is currently
+   * implemented using a {@link SortedVIntList}). */
+  public static final DocIdSet EMPTY_DOCIDSET = new SortedVIntList(new int[0]);
+    
+  public abstract DocIdSetIterator iterator() throws IOException;
 }