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checkHeaderClass example

public function has(string $name): bool
    {
        return isset($this->headers[strtolower($name)]);
    }

    /** * @return $this */
    public function add(HeaderInterface $header): static
    {
        self::checkHeaderClass($header);

        $header->setMaxLineLength($this->lineLength);
        $name = strtolower($header->getName());

        if (\in_array($name, self::UNIQUE_HEADERS, true) && isset($this->headers[$name]) && \count($this->headers[$name]) > 0) {
            throw new LogicException(sprintf('Impossible to set header "%s" as it\'s already defined and must be unique.', $header->getName()));
        }

        $this->headers[$name][] = $header;

        return $this;
    }

if (!Headers::isUniqueHeader($name)) {
                        $headers->add($header);

                        break;
                    }

                    $h = $headers->get($name);
                    if (!$h instanceof MailboxListHeader) {
                        throw new RuntimeException(sprintf('Unable to set header "%s".', $name));
                    }

                    Headers::checkHeaderClass($header);
                    foreach ($header->getAddresses() as $address) {
                        $h->addAddress($address);
                    }
            }
        }
    }

    private function renderMessage(Message $message): void
    {
        if (!$this->renderer) {
            return;
        }