code: def _find_new_additions(self):
          
          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)
docs: Find any nodes in the graph that need to be added to the internal
          queue and add them.
  
          Callers must hold the lock.