docs: def _joined_ids(self):
          """
          Adds the subjects/visits used to generate the derivatives iff there are
          any joins over them in the pipeline
          """
          joined_prov = {}
          if self.joins_subjects:
              joined_prov['subject_ids'] = list(self.study.subject_ids)
          if self.joins_visits:
              joined_prov['visit_ids'] = list(self.study.visit_ids)
          return joined_prov