code: def prune(self, objs=None):
          
          if objs is None:
              objdir = os.path.join(self._path, self.OBJ_DIR)
              objs = os.listdir(objdir)
          remove_objs = set(objs)
  
          for pkg in self.iterpackages():
              remove_objs.difference_update(find_object_hashes(pkg))
  
          for obj in remove_objs:
              path = self.object_path(obj)
              if os.path.exists(path):
                  os.chmod(path, S_IWUSR)
                  os.remove(path)
          return remove_objs
docs: Clean up objects not referenced by any packages. Try to prune all
          objects by default.