public function log ($level, $message, array $context = [])
  {
    $levelName = isset(self::$LEVELS[$level]) ? self::$LEVELS[$level] : $level;
    $message   = Debug::interpolate ($message, $context);
    $this->write ("<#log><#i><span class=__alert>$levelName</span> $message</#i></#log>");
  }