@Override
public DocSet union(DocSet other) {
OpenBitSet newbits = (OpenBitSet)(bits.clone());
if (other instanceof BitDocSet) {
newbits.union(((BitDocSet)other).bits);
} else {
DocIterator iter = other.iterator();
while (iter.hasNext()) newbits.set(iter.nextDoc());
}
return new BitDocSet(newbits);