Jyxo\Mail\Parser::isParentAlternative PHP Method

    private function isParentAlternative(int $partNo) : bool
    {
        // Multipart/alternative can be a child of only two types
        if ($this->structure['ftype'][$partNo] != 'text/plain' && $this->structure['ftype'][$partNo] != 'text/plain') {
            return false;
        }
        $partId = $this->structure['pid'][$partNo];
        $partLevel = count(explode('.', $partId));
        if (1 === $partLevel) {
            return $this->isPartMultipart(0, 'alternative');
        }
        $parentId = substr($partId, 0, strrpos($partId, '.'));
        for ($i = 0; $i < count($this->structure['pid']); $i++) {
            // There can be multiple parts with the same Id (because we assign parent Id to parts without an own Id)
            if ($parentId == $this->structure['pid'][$i] && $this->isPartMultipart($i, 'alternative')) {
                return true;
            }
        }
        return false;
    }