code: def to_pg_scheme(self, container=None, positions=None):
          if container is None and positions is None:
              raise Exception('electrode positions are required for BERT export')
          if container is not None and container.electrodes is None:
              raise Exception('container does not contain electrode positions')
          if container is not None and positions is not None:
              raise Exception(
                  'only one of container OR positions must be provided')
          if container is not None:
              elec_positions = container.electrodes.values
          elif positions is not None:
              elec_positions = positions
          opt_import("pybert", requiredFor="")
          import pybert
          data = pybert.DataContainerERT()
          for nr, (x, y, z) in enumerate(elec_positions):
              data.createSensor((x, y, z))
          data.resize(self.configs.shape[0])
          for index, token in enumerate("abmn"):
              data.set(token, self.configs[:, index].tolist())
          for token in "abmn":
              data.set(token, data(token) - 1)
          return data