String puid = frame_uid.toString() +attr+ val.toString(); this.hashcode = puid.hashCode();
	public Change(UID frame_uid, String attr, Object val)	{	    this.slot = attr;	    this.value = val;	    this.frame_uid = frame_uid;	    String puid = frame_uid.toString() +attr+ val.toString();	    this.hashcode = puid.hashCode();	}