code: def test_skip_constant():
      x = np.ones((100, 1))
      appended = add_trend(x, trend="c", has_constant="add")
      assert_array_equal(np.ones((100, 2)), appended)
      appended = add_trend(x, trend="c", has_constant="skip")
      assert_array_equal(np.ones((100, 1)), appended)
  
imports: import numpy
  from numpy.random import RandomState
  from numpy.testing import assert_array_almost_equal, assert_array_equal, assert_equal
  import pandas
  from pandas.testing import assert_frame_equal
  import pytest
  from arch.utility.timeseries import ColumnNameConflict, add_trend
label_desc: Use image node_id 12 for calling a global function with example usage: test_skip_constant() without return types