code: def node_width(self):
          
          nodes = self.postorder()
          node_width = {}
          for node in nodes:
              sum_width = 0
              for child in self[node].nodes():
                  sum_width += node_width[child]
              node_width[node] = (sum_width or 1)
          return node_width
docs: Returns the width of each node in the graph. #TODO