code: def _linkToParent(self, feature, parentName):
          
          parentParts = self.byFeatureName.get(parentName)
          if parentParts is None:
              raise GFF3Exception(
                  "Parent feature does not exist: {}".format(parentName),
                  self.fileName)
          # parent maybe disjoint
          for parentPart in parentParts:
              feature.parents.add(parentPart)
              parentPart.children.add(feature)
docs: Link a feature with its children