if(senders == null) return "[]";
    public String toString() {        StringBuffer sb=new StringBuffer();        boolean first=true;        for(int i=0; i < senders.length; i++) {            if(!first) {                sb.append(", ");            }            else {                sb.append("[");                first=false;            }            sb.append(senders[i]).append(": ").append("[").append(low_seqnos[i]).append(" : ");            sb.append(high_seqnos[i]);            if(high_seqnos_seen[i] >= 0)                sb.append(" (").append(high_seqnos_seen[i]).append(")]");        }        sb.append("]");        return sb.toString();    }