code: def _clean_snapshots(self):
      self.all_snapshots = [
          ref for ref in self.all_snapshots if ref() is not None
      ]
  
imports: import collections
  import functools
  import timeit
  import traceback
  import weakref
  from json import dumps
  from typing import List
  import numpy
  from .ascii_table import ascii_table
label_desc: Use image node_id 3 for calling the HierarchicalTimer obj's underlying member method code with example usage: obj._clean_snapshots() without return types