function lookup_nonce($consumer, $token, $nonce, $timestamp)
 {
     $n = new Nonce();
     $n->consumer_key = $consumer->key;
     $n->ts = common_sql_date($timestamp);
     $n->nonce = $nonce;
     if ($n->find(true)) {
         return true;
     } else {
         $n->created = DB_DataObject_Cast::dateTime();
         $n->insert();
         return false;
     }
 }