code: def prepare_series(self) -> None:
          # noinspection PyUnresolvedReferences
          
          memory = set()
          for output in itertools.chain(self.readers, self.writers):
              output.prepare_series(memory)
docs: Call |XMLSubseries.prepare_series| of all |XMLSubseries|
          objects with the same memory |set| object.
  
          >>> from hydpy.auxs.xmltools import XMLInterface, XMLSubseries
          >>> from hydpy import data
          >>> interface = XMLInterface('single_run.xml', data.get_path('LahnH'))
          >>> series_io = interface.series_io
          >>> from unittest import mock
          >>> prepare_series = XMLSubseries.prepare_series
          >>> XMLSubseries.prepare_series = mock.MagicMock()
          >>> series_io.prepare_series()
          >>> args = XMLSubseries.prepare_series.call_args_list
          >>> len(args) == len(series_io.readers) + len(series_io.writers)
          True
          >>> args[0][0][0]
          set()
          >>> args[0][0][0] is args[-1][0][0]
          True
          >>> XMLSubseries.prepare_series = prepare_series