code: def model2subs2seqs(self) -> Dict[str, Dict[str, List[str]]]:
          
          model2subs2seqs = collections.defaultdict(
              lambda: collections.defaultdict(list))
          for model in self.find('sequences'):
              model_name = strip(model.tag)
              if model_name == 'node':
                  continue
              for group in model:
                  group_name = strip(group.tag)
                  for sequence in group:
                      seq_name = strip(sequence.tag)
                      model2subs2seqs[model_name][group_name].append(seq_name)
          return model2subs2seqs
docs: A nested |collections.defaultdict| containing the model specific
          information provided by the XML `sequences` element.
  
          >>> from hydpy.auxs.xmltools import XMLInterface
          >>> from hydpy import data
          >>> interface = XMLInterface('single_run.xml', data.get_path('LahnH'))
          >>> series_io = interface.series_io
          >>> model2subs2seqs = series_io.writers[2].model2subs2seqs
          >>> for model, subs2seqs in sorted(model2subs2seqs.items()):
          ...     for subs, seq in sorted(subs2seqs.items()):
          ...         print(model, subs, seq)
          hland_v1 fluxes ['pc', 'tf']
          hland_v1 states ['sm']
          hstream_v1 states ['qjoints']