docs: def as_dfile(self):
          """
          Return the dataset representation in BMDS .(d) file.
          """
          rows = ["Dose Response"]
          for dose, response in zip(self.individual_doses, self.responses):
              dose_idx = self.doses.index(dose)
              if dose_idx >= self.num_dose_groups:
                  continue
              rows.append("%f %f" % (dose, response))
          return "\n".join(rows)