Index: lucene/core/src/java/org/apache/lucene/util/FixedBitSet.java
===================================================================
--- lucene/core/src/java/org/apache/lucene/util/FixedBitSet.java	(revision 1531190)
+++ lucene/core/src/java/org/apache/lucene/util/FixedBitSet.java	(working copy)
@@ -65,12 +65,16 @@
     this.bits = storedBits;
   }      
   
-  /** Makes full copy. */
-  public FixedBitSet(FixedBitSet other) {
-    bits = new long[other.wordLength];
-    System.arraycopy(other.bits, 0, bits, 0, other.wordLength);
-    numBits = other.numBits;
-    wordLength = other.wordLength;
+  /**
+   * Makes a full copy of the bits, while allowing to expand/shrink the bitset.
+   * If {@code numBits < other.numBits}, then only the first {@code numBits}
+   * are copied from other.
+   */
+  public FixedBitSet(FixedBitSet other, int numBits) {
+    wordLength = bits2words(numBits);
+    bits = new long[wordLength];
+    System.arraycopy(other.bits, 0, bits, 0, Math.min(other.wordLength, wordLength));
+    this.numBits = numBits;
   }
 
   @Override
@@ -403,7 +407,7 @@
 
   @Override
   public FixedBitSet clone() {
-    return new FixedBitSet(this);
+    return new FixedBitSet(this, numBits);
   }
 
   /** returns true if both sets have the same bits set */
Index: lucene/core/src/test/org/apache/lucene/util/TestFixedBitSet.java
===================================================================
--- lucene/core/src/test/org/apache/lucene/util/TestFixedBitSet.java	(revision 1531190)
+++ lucene/core/src/test/org/apache/lucene/util/TestFixedBitSet.java	(working copy)
@@ -328,7 +328,50 @@
     
     checkNextSetBitArray(new int[0], setBits.length + random().nextInt(10));
   }
-}
+  
+  public void testGrow() {
+    FixedBitSet bits = new FixedBitSet(5);
+    bits.set(1);
+    bits.set(4);
+    
+    FixedBitSet newBits = new FixedBitSet(bits, 8); // grow within the word
+    assertTrue(newBits.get(1));
+    assertTrue(newBits.get(4));
 
+    newBits = new FixedBitSet(bits, 72); // grow beyond one word
+    assertTrue(newBits.get(1));
+    assertTrue(newBits.get(4));
+  }
+  
+  public void testShrink() {
+    FixedBitSet bits = new FixedBitSet(72);
+    bits.set(1);
+    bits.set(4);
+    bits.set(69);
+    
+    FixedBitSet newBits = new FixedBitSet(bits, 66); // shrink within the word
+    assertTrue(newBits.get(1));
+    assertTrue(newBits.get(4));
+    boolean hitError = true;
+    try {
+      newBits.get(69);
+      hitError = false;
+    } catch (AssertionError e) {
+      hitError = true;
+    }
+    assertTrue(hitError);
 
-
+    newBits = new FixedBitSet(bits, 8); // shrink beyond one word
+    assertTrue(newBits.get(1));
+    assertTrue(newBits.get(4));
+    hitError = true;
+    try {
+      newBits.get(69);
+      hitError = false;
+    } catch (AssertionError e) {
+      hitError = true;
+    }
+    assertTrue(hitError);
+  }
+  
+}