code: def test_len(self):
      s = self.cls()
      self.assertEmpty(s)
      for i in range(10):
          self.assertLen(s, i)
          s.push(None)
      for i in range(10):
          self.assertLen(s, 10 - i)
          s.pop()
      self.assertEmpty(s)
  
imports: import collections
  import copy
  import pickle
  import threading
  from absl.testing import absltest
  from absl.testing import parameterized
  import cloudpickle
  import dill
  from haiku._src import data_structures
  import jax
  import tree
label_desc: Use image node_id 1 for calling the StackTest obj's underlying member method code with example usage: obj.test_len() without return types