@Override
   public boolean retainAll(Collection<?> c) {
      throw new UnsupportedOperationException("RangeSet is immutable");
   }
@Test(expected = UnsupportedOperationException.class)
   public void retainAll() throws Exception {
      RangeSet rs = new RangeSet(4);

      RangeSet rs2 = new RangeSet(5);

      rs.retainAll(rs2);
   }