code: def gen_reciprocals(self, append=False):
          reciprocals = self.configs.copy()[:, ::-1]
          reciprocals[:, 0:2] = np.sort(reciprocals[:, 0:2], axis=1)
          reciprocals[:, 2:4] = np.sort(reciprocals[:, 2:4], axis=1)
          ind = np.lexsort((reciprocals[:, 3], reciprocals[:, 2],
                            reciprocals[:, 1], reciprocals[:, 0]))
          reciprocals = reciprocals[ind]
          if append:
              self.configs = np.vstack((self.configs, reciprocals))
          return reciprocals