docs: def _collect_pathways_by_feature(self, pathway_feature_tuples):
          """Given a tuple list [(pathway, feature)], create a dictionary
          mapping each feature to the pathways overrepresented in the feature.
          """
          pathways_in_feature = {}
          for (pathway, feature) in pathway_feature_tuples:
              vertex = self.add_pathway(pathway)
              if feature not in pathways_in_feature:
                  pathways_in_feature[feature] = []
              pathways_in_feature[feature].append(vertex)
          return pathways_in_feature