code: def _warn_missing_chs(info, dig_image, after_warp=False):
      """Warn that channels are missing."""
      # ensure that each electrode contact was marked in at least one voxel
      missing = set(np.arange(1, len(info.ch_names) + 1)).difference(
          set(np.unique(np.array(dig_image.dataobj)))
      )
      missing_ch = [info.ch_names[idx - 1] for idx in missing]
      if missing_ch:
          warn(
              f"Channel{_pl(missing_ch)} "
              f'{", ".join(repr(ch) for ch in missing_ch)} not assigned '
              "voxels "
              + (f" after applying {after_warp}" if after_warp else "")
          )
  
imports: import numpy
  from ...channels import DigMontage, make_dig_montage
  from ...surface import _voxel_neighbors
  from ...transforms import Transform, _frame_to_str, apply_trans
  from ...utils import _check_option, _pl, _require_version, _validate_type, verbose, warn
label_desc: Use image node_id 2 for calling a global function with example usage: _warn_missing_chs(info, dig_image, after_warp) without return types