docs: def augment_cycle(self, amount, cycle):
          '''
          API:
              augment_cycle(self, amount, cycle):
          Description:
              Augments 'amount' unit of flow along cycle.
          Pre:
              Arcs should have 'flow' attribute.
          Inputs:
              amount: An integer representing the amount to augment
              cycle: A list representing a cycle
          Post:
              Changes 'flow' attributes of arcs.
          '''
          index = 0
          k = len(cycle)
          while index<(k-1):
              i = cycle[index]
              j = cycle[index+1]
              if (i,j) in self.edge_attr:
                  flow_ij = self.edge_attr[(i,j)]['flow']
                  self.edge_attr[(i,j)]['flow'] = flow_ij+amount
              else:
                  flow_ji = self.edge_attr[(j,i)]['flow']
                  self.edge_attr[(j,i)]['flow'] = flow_ji-amount
              index += 1
          i = cycle[k-1]
          j = cycle[0]
          if (i,j) in self.edge_attr:
              flow_ij = self.edge_attr[(i,j)]['flow']
              self.edge_attr[(i,j)]['flow'] = flow_ij+amount
          else:
              flow_ji = self.edge_attr[(j,i)]['flow']
              self.edge_attr[(j,i)]['flow'] = flow_ji-amount