code: def add_to(self, parent, additions):
          "Modify parent to include all elements in additions"
          for x in additions:
              if x not in parent:
                  parent.append(x)
                  self.changed(f add_to(self, parent, additions):
          "Modify parent to include all elements in additions"
          for x in additions:
              if x not in parent:
                  parent.append(x)
                  self.changed()
docs: Modify parent to include all elements in additions