code: def test_csd_pick_frequency():
      """Test the pick_frequency method of CrossSpectralDensity."""
      csd = _make_csd()
  
      csd2 = csd.pick_frequency(freq=2)
      assert csd2.frequencies == [2]
      assert_array_equal(
          csd2.get_data(), [[6, 7, 8], [7, 9, 10], [8, 10, 11]]
      )
  
      csd2 = csd.pick_frequency(index=1)
      assert csd2.frequencies == [2]
      assert_array_equal(
          csd2.get_data(), [[6, 7, 8], [7, 9, 10], [8, 10, 11]]
      )
  
      # Nonexistent frequency
      raises(IndexError, csd.pick_frequency, -1)
  
      # Nonexistent index
      raises(IndexError, csd.pick_frequency, index=10)
  
      # Invalid parameters
      raises(ValueError, csd.pick_frequency)
      raises(ValueError, csd.pick_frequency, freq=2, index=1)
  
imports: import pickle
  from itertools import product
  from os import path
  import numpy
  import pytest
  from numpy.testing import assert_allclose, assert_array_equal
  from pytest import raises
  import mne
  from mne.channels import equalize_channels
  from mne.proj import Projection
  from mne.time_frequency import CrossSpectralDensity, csd_array_fourier, csd_array_morlet, csd_array_multitaper, csd_fourier, csd_morlet, csd_multitaper, csd_tfr, pick_channels_csd, read_csd, tfr_morlet
  from mne.time_frequency.csd import _sym_mat_to_vector, _vector_to_sym_mat
  from mne.utils import sum_squared
label_desc: Use image node_id 6 for calling a global function with example usage: test_csd_pick_frequency() without return types