code: def write_psd(self, psds, group=None):
          
          subgroup = self.data_group + "/{ifo}/psds/0"
          if group is None:
              group = subgroup
          else:
              group = '/'.join([group, subgroup])
          for ifo in psds:
              self[group.format(ifo=ifo)] = psds[ifo]
              self[group.format(ifo=ifo)].attrs['delta_f'] = psds[ifo].delta_f
docs: Writes PSD for each IFO to file.
  
          Parameters
          -----------
          psds : {dict, FrequencySeries}
              A dict of FrequencySeries where the key is the IFO.
          group : {None, str}
              The group to write the psd to. Default is ``data_group``.