code: def dependency(self, node1, node2):
          "indicate that node1 depends on node2"
          self.graph.add_node(node1)
          self.graph.add_node(node2)
          self.graph.add_edge(node2, node1f dependency(self, node1, node2):
          "indicate that node1 depends on node2"
          self.graph.add_node(node1)
          self.graph.add_node(node2)
          self.graph.add_edge(node2, node1)
docs: indicate that node1 depends on node2