code: def test_pick_ref():
      """Test picking ref_meg channels."""
      info = read_info(ctf_fname)
      picks_by_type = [
          ("stim", [0]),
          ("eog", [306, 307]),
          ("ecg", [308]),
          ("misc", [1]),
          ("mag", np.arange(31, 306)),
          ("ref_meg", np.arange(2, 31)),
      ]
      assert_indexing(info, picks_by_type, all_data=False)
      picks_by_type.append(
          (
              "mag",
              np.concatenate(
                  [picks_by_type.pop(-1)[1], picks_by_type.pop(-1)[1]]
              ),
          )
      )
      assert_indexing(info, picks_by_type, ref_meg=True, all_data=False)
  
imports: from copy import deepcopy
  from pathlib import Path
  import numpy
  import pytest
  from numpy.testing import assert_array_equal
  from mne import Epochs, channel_indices_by_type, channel_type, create_info, make_ad_hoc_cov, pick_channels, pick_channels_cov, pick_channels_regexp, pick_info, pick_types, pick_types_forward, read_forward_solution, rename_channels
  from mne._fiff.constants import FIFF
  from mne._fiff.pick import _DATA_CH_TYPES_SPLIT, _contains_ch_type, _picks_by_type, _picks_to_idx, get_channel_type_constants
  from mne.channels import make_standard_montage
  from mne.datasets import testing
  from mne.io import RawArray, read_info, read_raw_bti, read_raw_fif, read_raw_kit
  from mne.preprocessing import compute_current_source_density
  from mne.utils import assert_object_equal, catch_logging
label_desc: Use image node_id 13 for calling a global function with example usage: test_pick_ref() without return types