def at!(path, node = doc)
      node.at(path) or raise Error::NodeNotFound, "node for `#{path}` not found"
    end
def at!(path, node = doc)
      node.at(path) or raise Error::NodeNotFound, "node for `#{path}` not found"
    end
+node.at(path)+ or raise error
 @param [String] path XPath or CSS path
 @return [Nokogiri::XML::Node]