observable.retainAll( ListFactory.unmodifiable( "a", "c" ) );
assertEquals( ListFactory.unmodifiable( "a", "c" ), base );
assertEquals( 1, log.size() );
log.clear();
observable.retainAll( ListFactory.unmodifiable( "a", "c" ) );
assertEquals( ListFactory.unmodifiable( "a", "c" ), base );
assertEquals( 0, log.size() );
}