protected function includeExternalSchema(\DOMElement $externalDatabaseElement)
    {
        $database = $this->getDatabaseFromDatabaseElement($externalDatabaseElement);

        $this->initGlobalDatabaseElement($database);

        $globalDatabaseElement = $this->globalDatabaseElements[$database];

        // add an inclusion notice
        $externalSchemaIncludeComment = $globalDatabaseElement->ownerDocument->createComment(
            "External schema included in the combining process"
        );
        $globalDatabaseElement->appendChild($externalSchemaIncludeComment);

        // include the external schema
        $externalSchemaInclude = $globalDatabaseElement->ownerDocument->createElement(
            'external-schema',
            $externalDatabaseElement->ownerDocument->baseURI
        );
        $globalDatabaseElement->appendChild($externalSchemaInclude);

        $this->externalSchemaDatabaseElements[$database][] = $externalDatabaseElement;
    }