code: def test_push_state_single():
      """
      Test that stacks are being updated correctly when using a single stack
  
      Values of the attention are not verified, though
      """
      ts = TransformerTreeStack(3, 5, 0.0)
      initial = ts.initial_state()
      rand_input = torch.randn(1, 3)
      stacks = ts.push_states([initial], ["A"], rand_input)
      stacks = ts.push_states(stacks, ["B"], rand_input)
      assert len(stacks) == 1
      assert len(stacks[0]) == 3
      assert stacks[0].value.value == "B"
      assert stacks[0].pop().value.value == "A"
      assert stacks[0].pop().pop().value.value is None
  
file_path: stanza/stanza/tests/constituency/test_transformer_tree_stack.py
imports: import pytest
  import torch
  from stanza.models.constituency.transformer_tree_stack import TransformerTreeStack
label_desc: Use image node_id 3 for calling a global function with example usage: test_push_state_single() without return types