code: def test_pandas_const_series_prepend():
      dta = longley.load_pandas()
      series = dta.exog["GNP"]
      series = tools.add_constant(series, prepend=True)
      assert_string_equal("const", series.columns[0])
      assert_equal(series.var(0).iloc[0], 0)
  
imports: from statsmodels.compat.pandas import assert_frame_equal, assert_series_equal
  from statsmodels.compat.python import lrange
  import string
  import numpy
  from numpy.random import standard_normal
  from numpy.testing import assert_almost_equal, assert_array_equal, assert_equal, assert_string_equal
  import pandas
  import pytest
  from statsmodels.datasets import longley
  from statsmodels.tools import tools
  from statsmodels.tools.tools import pinv_extended
label_desc: Use image node_id 4 for calling a global function with example usage: test_pandas_const_series_prepend() without return types