code: def test_len():
      stack = TreeStack(value=5, parent=None, length=1)
      assert len(stack) == 1
  
      stack = stack.push(3)
      stack = stack.push(1)
      assert len(stack) == 3
  
imports: import pytest
  from stanza.models.constituency.tree_stack import TreeStack
  from stanza.tests import *
label_desc: Use image node_id 4 for calling a global function with example usage: test_len() without return types