private void _checkKeys( DBObject o ) {
        if ( o instanceof LazyDBObject || o instanceof LazyDBList )
            return;

        for ( String s : o.keySet() ){
            validateKey ( s );
            Object inner = o.get( s );
            if ( inner instanceof DBObject ) {
                _checkKeys( (DBObject)inner );
            } else if ( inner instanceof Map ) {
                _checkKeys( (Map<String, Object>)inner );
            }
        }
    }