public function getHistory($sNamespace = null, $sEvent = null)
    {
        $sEvent     = strtoupper($sEvent);
        $sNamespace = strtoupper($sNamespace);

        if (empty($sNamespace) && empty($sEvent)) {
            return $this->aHistory;
        } elseif (
            empty($sEvent) &&
            !empty($sNamespace) &&
            array_key_exists($sNamespace, $this->aHistory)
        ) {
            return $this->aHistory[$sNamespace];
        } elseif (
            !empty($sNamespace) &&
            array_key_exists($sNamespace, $this->aHistory) &&
            !empty($sEvent) &&
            array_key_exists($sEvent, $this->aHistory[$sNamespace])
        ) {
            return $this->aHistory[$sNamespace][$sEvent];
        }

        return null;
    }