public function timestamp($domain, $where, $comment = null)
    {
        $now = microtime(true);
        $this->timestamp[] = array(
          'domain'  => $domain,
          'where'   => $where,
          'comment' => $comment,
          'when'    => $now,
          'memory'  => memory_get_usage(true),
          'duration'=> 0,
        );

        if ($this->pos > 0) {
            $this->timestamp[$this->pos - 1]['memory-peak'] = memory_get_peak_usage(true);
            $this->timestamp[$this->pos - 1]['duration']    = $now - $this->timestamp[$this->pos - 1]['when'];
        }
    
        $this->pos++;
    }