diff --git a/CHANGES b/CHANGES
index 6b7e832..53182d2 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,7 @@
+0.21 2013-11-28
+        - Virtual collections can now be defined,
+	  searched, nested and extended.
+
 0.20 2013-11-18
         - Unboxing from sandbox.
 
diff --git a/pom.xml b/pom.xml
index e127b7b..f7b05f3 100644
--- a/pom.xml
+++ b/pom.xml
@@ -9,7 +9,7 @@
 
   <groupId>KorAP-modules</groupId>
   <artifactId>KorAP-lucene-index</artifactId>
-  <version>0.20</version>
+  <version>0.21</version>
   <packaging>jar</packaging>
 
   <name>KorAP-lucene-index</name>
diff --git a/src/main/java/de/ids_mannheim/korap/KorapCollection.java b/src/main/java/de/ids_mannheim/korap/KorapCollection.java
index a28800d..97e61d6 100644
--- a/src/main/java/de/ids_mannheim/korap/KorapCollection.java
+++ b/src/main/java/de/ids_mannheim/korap/KorapCollection.java
@@ -10,6 +10,7 @@
 import de.ids_mannheim.korap.KorapFilter;
 import de.ids_mannheim.korap.util.KorapDate;
 import de.ids_mannheim.korap.filter.BooleanFilter;
+import de.ids_mannheim.korap.filter.FilterOperation;
 import org.apache.lucene.search.spans.SpanQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.FilteredQuery;
@@ -26,6 +27,8 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+// TODO: Make a cache for the bits!!! DELETE IT IN CASE OF AN EXTENSION OR A FILTER!
+
 
 // accepts as first parameter the index
 // THIS MAY CHANGE for stuff like combining virtual collections
@@ -35,23 +38,7 @@
     private KorapIndex index;
     private String id;
     private KorapDate created;
-
-
-    // TODO:
-    // Change this to support join operation
-    /*
-    private class CollectionOperation () {
-	private boolean type;
-	private Filter filter;
-	private CollectionOperation (type, filter) {
-	    this.type = type;
-	    this.filter = filter;
-	};
-    };
-    */
-
-    private ArrayList<Filter> filter;
-
+    private ArrayList<FilterOperation> filter;
     private int filterCount = 0;
     
     // Logger
@@ -60,7 +47,7 @@
     // user?
     public KorapCollection (KorapIndex ki) {
 	this.index = ki;
-	this.filter = new ArrayList<Filter>(5);
+	this.filter = new ArrayList<FilterOperation>(5);
     };
 
     public int getCount() {
@@ -68,11 +55,26 @@
     };
 
     public void filter (BooleanFilter filter) {
-	this.filter.add(new QueryWrapperFilter(filter.toQuery()));
+	this.filter.add(
+	    new FilterOperation(
+				(Filter) new QueryWrapperFilter(filter.toQuery()),
+                false
+            )
+        );
 	this.filterCount++;
     };
 
-    public ArrayList<Filter> getFilters () {
+    public void extend (BooleanFilter filter) {
+	this.filter.add(
+	    new FilterOperation(
+				(Filter) new QueryWrapperFilter(filter.toQuery()),
+                true
+            )
+        );
+	this.filterCount++;
+    };
+
+    public ArrayList<FilterOperation> getFilters () {
 	return this.filter;
     };
 
@@ -96,10 +98,10 @@
 	if (this.filterCount > 0) {
 	    bitset = new FixedBitSet(atomic.reader().numDocs());
 
-	    ArrayList<Filter> filters = (ArrayList<Filter>) this.filter.clone();
+	    ArrayList<FilterOperation> filters = (ArrayList<FilterOperation>) this.filter.clone();
 
 	    // Init vector
-	    DocIdSet docids = filters.remove(0).getDocIdSet(atomic, null);
+	    DocIdSet docids = filters.remove(0).filter.getDocIdSet(atomic, null);
 	    DocIdSetIterator filterIter = docids.iterator();
 
 	    if (filterIter != null) {
@@ -108,17 +110,27 @@
 	    };
 
 	    if (!noDoc) {
-		for (Filter kc : filters) {
+		for (FilterOperation kc : filters) {
 		    log.trace("FILTER: {}", kc);
-		    docids = kc.getDocIdSet(atomic, bitset);
+
+		    // BUG!!!
+		    docids = kc.filter.getDocIdSet(atomic, kc.isExtension() ? null : bitset);
 		    filterIter = docids.iterator();
+
 		    if (filterIter == null) {
 			// There must be a better way ...
-			bitset.clear(0, bitset.length());
-			noDoc = true;
-			break;
+			if (kc.isFilter()) {
+			    bitset.clear(0, bitset.length());
+			    noDoc = true;
+			};
+			continue;
 		    };
-		    bitset.and(filterIter);
+		    if (kc.isExtension()) {
+			bitset.or(filterIter);
+		    }
+		    else {
+			bitset.and(filterIter);
+		    };
 		};
 
 		if (!noDoc) {
diff --git a/src/main/java/de/ids_mannheim/korap/filter/FilterOperation.java b/src/main/java/de/ids_mannheim/korap/filter/FilterOperation.java
new file mode 100644
index 0000000..20dde1c
--- /dev/null
+++ b/src/main/java/de/ids_mannheim/korap/filter/FilterOperation.java
@@ -0,0 +1,38 @@
+package de.ids_mannheim.korap.filter;
+import org.apache.lucene.search.Filter;
+
+public class FilterOperation {
+    private boolean extension;
+    public Filter filter;
+    
+    public FilterOperation (Filter filter, boolean extension) {
+	this.extension = extension;
+	this.filter = filter;
+    };
+
+    public boolean isExtension () {
+	return this.extension;
+    };
+
+    public boolean isFilter () {
+	return !(this.extension);
+    };
+
+    @Override
+    public Object clone () throws CloneNotSupportedException {
+	return (Object) new FilterOperation(this.filter, this.extension);
+    };
+
+    @Override
+    public String toString () {
+	StringBuilder sb = new StringBuilder();
+	if (this.extension) {
+	    sb.append("extend with ");
+	}
+	else {
+	    sb.append("filter with ");
+	};
+	sb.append(this.filter.toString());
+	return sb.toString();
+    };	
+};
diff --git a/src/test/java/de/ids_mannheim/korap/filter/TestKorapCollection.java b/src/test/java/de/ids_mannheim/korap/filter/TestKorapCollection.java
index ed1101e..2e557b4 100644
--- a/src/test/java/de/ids_mannheim/korap/filter/TestKorapCollection.java
+++ b/src/test/java/de/ids_mannheim/korap/filter/TestKorapCollection.java
@@ -59,8 +59,15 @@
 	KorapResult kr = kc.search(query);
 	assertEquals(70, kr.totalResults());
 
+	kc.extend( kf.and("textClass", "uninteresting") );
+	assertEquals("Documents", 1, kc.numberOf("documents"));
 
-	// System.err.println(kr.toJSON());
+	kc.extend( kf.and("textClass", "wissenschaft") );
+	assertEquals("Documents", 3, kc.numberOf("documents"));
+	assertEquals("Tokens", 1669, kc.numberOf("tokens"));
+	assertEquals("Sentences", 188, kc.numberOf("sentences"));
+	assertEquals("Paragraphs", 130, kc.numberOf("paragraphs"));
+	System.err.println(kr.toJSON());
     };
 };