code: def test_vcomp_3(self):
      # Test a model with vcomp but no other random effects, using formulas.
  
      np.random.seed(4279)
      x1 = np.random.normal(size=400)
      groups = np.kron(np.arange(100), np.ones(4))
      slopes = np.random.normal(size=100)
      slopes = np.kron(slopes, np.ones(4)) * x1
      y = slopes + np.random.normal(size=400)
      vc_fml = {"a": "0 + x1"}
      df = pd.DataFrame({"y": y, "x1": x1, "groups": groups})
  
      model = MixedLM.from_formula(
          "y ~ 1", groups="groups", vc_formula=vc_fml, data=df
      )
      result = model.fit()
      result.summary()
  
      assert_allclose(
          result.resid.iloc[0:4],
          np.r_[-1.180753, 0.279966, 0.578576, -0.667916],
          rtol=1e-3,
      )
      assert_allclose(
          result.fittedvalues.iloc[0:4],
          np.r_[-0.101549, 0.028613, -0.224621, -0.126295],
          rtol=1e-3,
      )
  
imports: from statsmodels.compat.platform import PLATFORM_OSX
  import os
  import csv
  import warnings
  import numpy
  import pandas
  from scipy import sparse
  import pytest
  from statsmodels.regression.mixed_linear_model import MixedLM, MixedLMParams, _smw_solver, _smw_logdet
  from numpy.testing import assert_almost_equal, assert_equal, assert_allclose, assert_
  from statsmodels.base import _penalties
  import statsmodels.tools.numdiff
  from .results import lme_r_results
label_desc: Use image node_id 7 for calling the TestMixedLM obj's underlying member method code with example usage: obj.test_vcomp_3() without return types