code: public function add(string $header): self
      {
          foreach ($this->getAll() as $tmp) {
              if ($tmp === $header) {
                  throw new Exception("The '{$header}' header has already been added.");
              }
          }
          $this->headerList[] = $header;
          return self::$instance;
      }
docs: It adds the header to the stack.
  @param string $header The page header.
  @return self.
  @throws \nadir2\core\Exception It's thrown if the passed header has already
  been added earlier.