Find any nodes in the graph that need to be added to the internal
        queue and add them.

        Callers must hold the lock.

def _find_new_additions(self):
        """Find any nodes in the graph that need to be added to the internal
        queue and add them.

        Callers must hold the lock.
        """
        for node, in_degree in self.graph.in_degree_iter():
            if not self._already_known(node) and in_degree == 0:
                self.inner.put((self._scores[node], node))
                self.queued.add(node)