docs: def iterators(self, frequency=None):
          """
          Returns the iterators (i.e. subject_id, visit_id) that the pipeline
          iterates over
  
          Parameters
          ----------
          frequency : str | None
              A selected data frequency to use to determine which iterators are
              required. If None, all input frequencies of the pipeline are
              assumed
          """
          iterators = set()
          if frequency is None:
              input_freqs = list(self.input_frequencies)
          else:
              input_freqs = [frequency]
          for freq in input_freqs:
              iterators.update(self.study.FREQUENCIES[freq])
          return iterators