code: def test_iforest_warm_start():
      """Test iterative addition of iTrees to an iForest"""
  
      rng = check_random_state(0)
      X = rng.randn(20, 2)
  
      # fit first 10 trees
      clf = IsolationForest(
          n_estimators=10,
          max_samples=20,
          random_state=rng,
          warm_start=True,
      )
      clf.fit(X)
      # remember the 1st tree
      tree_1 = clf.estimators_[0]
      # fit another 10 trees
      clf.set_params(n_estimators=20)
      clf.fit(X)
      # expecting 20 fitted trees and no overwritten trees
      assert len(clf.estimators_) == 20
      assert clf.estimators_[0] is tree_1
  
imports: import warnings
  from unittest.mock import Mock, patch
  import numpy
  import pytest
  from sklearn.datasets import load_diabetes, load_iris, make_classification
  from sklearn.ensemble import IsolationForest
  from sklearn.ensemble._iforest import _average_path_length
  from sklearn.metrics import roc_auc_score
  from sklearn.model_selection import ParameterGrid, train_test_split
  from sklearn.utils import check_random_state
  from sklearn.utils._testing import assert_allclose, assert_array_almost_equal, assert_array_equal, ignore_warnings
  from sklearn.utils.fixes import CSC_CONTAINERS, CSR_CONTAINERS
label_desc: Use image node_id 13 for calling a global function with example usage: test_iforest_warm_start() without return types