docs: def _save_action(extra_context=None):
      '''
      Save list of revisions revisions for active Conda environment.
  
      .. versionchanged:: 0.18
          Compress action revision files using ``bz2`` to save disk space.
  
      Parameters
      ----------
      extra_context : dict, optional
          Extra content to store in stored action revision.
  
      Returns
      -------
      path_helpers.path, dict
          Path to which action was written and action object, including list of
          revisions for active Conda environment.
      '''
      # Get list of revisions to Conda environment since creation.
      revisions_js = ch.conda_exec('list', '--revisions', '--json',
                                   verbose=False)
      revisions = json.loads(revisions_js)
      # Save list of revisions to `/etc/microdrop/plugins/actions/rev<rev>.json`
      # See [wheeler-microfluidics/microdrop#200][i200].
      #
      # [i200]: https://github.com/wheeler-microfluidics/microdrop/issues/200
      action = extra_context.copy() if extra_context else {}
      action['revisions'] = revisions
      action_path = (MICRODROP_CONDA_ACTIONS
                     .joinpath('rev{}.json.bz2'.format(revisions[-1]['rev'])))
      action_path.parent.makedirs_p()
      # Compress action file using bz2 to save disk space.
      with bz2.BZ2File(action_path, mode='w') as output:
          json.dump(action, output, indent=2)
      return action_path, action