if(callStack != null) ret.append(", call stack=" + callStack);
        public String toString() {            StringBuffer ret=new StringBuffer();            ret.append("[Header: name=" + corrName + ", type=");            ret.append(type == REQ ? "REQ" : type == RSP ? "RSP" : "<unknown>");            ret.append(", id=" + id);            ret.append(", rsp_expected=" + rsp_expected + ']');            if(dest_mbrs != null)                ret.append(", dest_mbrs=").append(dest_mbrs);            return ret.toString();        }