convert (key, value) stored in this and the descendant nodes
        to dict items.

        :param node: node in form of list, or BLANK_NODE

        .. note::

            Here key is in full form, rather than key of the individual node

def _to_dict(self, node):
        """convert (key, value) stored in this and the descendant nodes
        to dict items.

        :param node: node in form of list, or BLANK_NODE

        .. note::

            Here key is in full form, rather than key of the individual node
        """
        if node == BLANK_NODE:
            return {}

        node_type = self._get_node_type(node)

        if is_key_value_type(node_type):
            nibbles = without_terminator(unpack_to_nibbles(node[0]))
            key = b'+'.join([to_string(x) for x in nibbles])
            if node_type == NODE_TYPE_EXTENSION:
                sub_dict = self._to_dict(self._decode_to_node(node[1]))
            else:
                sub_dict = {to_string(NIBBLE_TERMINATOR): node[1]}

            # prepend key of this node to the keys of children
            res = {}
            for sub_key, sub_value in sub_dict.items():
                full_key = (key + b'+' + sub_key).strip(b'+')
                res[full_key] = sub_value
            return res

        elif node_type == NODE_TYPE_BRANCH:
            res = {}
            for i in range(16):
                sub_dict = self._to_dict(self._decode_to_node(node[i]))

                for sub_key, sub_value in sub_dict.items():
                    full_key = (
                        str_to_bytes(
                            str(i)) +
                        b'+' +
                        sub_key).strip(b'+')
                    res[full_key] = sub_value

            if node[16]:
                res[to_string(NIBBLE_TERMINATOR)] = node[-1]
            return res