code: def prepare_series(self, memory: set) -> None:
          
          for sequence in self._iterate_sequences():
              if sequence not in memory:
                  memory.add(sequence)
                  sequence.activate_ram()
docs: Call |IOSequence.activate_ram| of all sequences selected by
          the given output element of the actual XML file.
  
          Use the memory argument to pass in already prepared sequences;
          newly prepared sequences will be added.
  
          >>> from hydpy.core.examples import prepare_full_example_1
          >>> prepare_full_example_1()
  
          >>> from hydpy import HydPy, TestIO, XMLInterface
          >>> hp = HydPy('LahnH')
          >>> with TestIO():
          ...     hp.prepare_network()
          ...     hp.init_models()
          ...     interface = XMLInterface('single_run.xml')
          >>> interface.update_timegrids()
          >>> series_io = interface.series_io
  
          >>> memory = set()
          >>> pc = hp.elements.land_dill.model.sequences.fluxes.pc
          >>> pc.ramflag
          False
          >>> series_io.writers[0].prepare_series(memory)
          >>> pc in memory
          True
          >>> pc.ramflag
          True
  
          >>> pc.deactivate_ram()
          >>> pc.ramflag
          False
          >>> series_io.writers[0].prepare_series(memory)
          >>> pc.ramflag
          False