public function inCategory(string $category, string $domain = ''): self
    {
        $this->categories[] = ['category' => $category, 'domain'   => $domain];
        return $this;
    }
public function inCategory(string $category, string $domain = ''): self
    {
        $this->categories[] = ['category' => $category, 'domain'   => $domain];
        return $this;
    }
public function inCategory ( string $ category , string $ domain = '' ) : self { $ this -> categories [ ] = [ 'category' => $ category , 'domain' => $ domain ] ; return $ this ; }
set one or more categories where the item is included into

@param   string  $category  category where the item is included
@param   string  $domain    categorization taxonomy
@return  \stubbles\xml\rss\RssFeedItem
set one or more categories where the item is included into