def exist_node(self, graph, node, branch, turn, tick, extant):
        """Declare that the node exists or doesn't.

        Inserts a new record or updates an old one, as needed.

        """
        if (branch, turn, tick) in self._btts:
            raise TimeError
        self._btts.add((branch, turn, tick))
        self._nodes2set.append((self.pack(graph), self.pack(node), branch, turn, tick, extant))
Declare that the node exists or doesn't.

        Inserts a new record or updates an old one, as needed.