Get value of a key when the root node was `root_node`
        :param root_node:
        :param key:
        :return:

def get_at(self, root_node, key):
        """
        Get value of a key when the root node was `root_node`
        :param root_node:
        :param key:
        :return:
        """
        return self._get(root_node, bin_to_nibbles(to_string(key)))