code: def _set_montage_coreg_path(subjects_dir=None):
      """Set a subject directory suitable for montage(-only) coregistration.
  
      Parameters
      ----------
      subjects_dir : path-like | None
          The path to use as the subjects directory in the MNE-Python
          config file. None will use the existing config variable (i.e.,
          will not change anything), and if it does not exist, will use
          ``~/mne_data/MNE-fsaverage-data``.
  
      Returns
      -------
      subjects_dir : str
          The subjects directory that was used.
  
      See Also
      --------
      mne.datasets.fetch_fsaverage
      mne.get_config
      mne.set_config
  
      Notes
      -----
      If you plan to only do EEG-montage based coregistrations with fsaverage
      without any MRI warping, this function can facilitate the process.
      Essentially it sets the default value for ``subjects_dir`` in MNE
      functions to be ``~/mne_data/MNE-fsaverage-data`` (assuming it has
      not already been set to some other value).
  
      .. versionadded:: 0.18
      """
      subjects_dir = _get_create_subjects_dir(subjects_dir)
      old_subjects_dir = get_subjects_dir(None, raise_error=False)
      if old_subjects_dir is None:
          set_config("SUBJECTS_DIR", subjects_dir)
      return subjects_dir
  
imports: import os
  import os.path
  from ...utils import get_subjects_dir, set_config, verbose
  from ..utils import _get_path, _manifest_check_download
label_desc: Use image node_id 3 for calling a global function with example usage: _set_montage_coreg_path(subjects_dir) and returns: subjects_dir