public function flattenedMappings(): array
    {
        $mappings = array();
        foreach ($this->_mappings as $mapping) {
            $idp = $mapping->issuerDomainPolicy();
            if (!isset($mappings[$idp])) {
                $mappings[$idp] = array();
            }
            array_push($mappings[$idp], $mapping->subjectDomainPolicy());
        }
        return $mappings;
    }