protected function projectHeaders()
    {
        foreach ($this->getStaticHeaders() as $header) {
            yield $header;
        }

        foreach ($this->getAttributeKeysAndControllers() as list($attributeKey, $attributeController)) {
            /* @var \Concrete\Core\Attribute\AttributeKeyInterface $attributeKey */
            /* @var \Concrete\Core\Attribute\Controller $attributeController */
            $handle = 'a:' . $attributeKey->getAttributeKeyHandle();
            if ($attributeController instanceof SimpleTextExportableAttributeInterface) {
                yield $handle;
            } elseif ($attributeController instanceof MulticolumnTextExportableAttributeInterface) {
                foreach ($attributeController->getAttributeTextRepresentationHeaders() as $subHeader) {
                    yield $handle . '[' . $subHeader . ']';
                }
            }
        }
    }