docs: def as_dfile(self):
          """
          Return the dataset representation in BMDS .(d) file.
          """
          rows = ["Dose NumAnimals Response Stdev"]
          for i, v in enumerate(self.doses):
              if i >= self.num_dose_groups:
                  continue
              rows.append(
                  "%f %d %f %f" % (self.doses[i], self.ns[i], self.means[i], self.stdevs[i])
              )
          return "\n".join(rows)