docs: def extract(group_id, access_token, fields=None):
      '''
      FIXME: DOCS...
      Links:
         * https://developers.facebook.com/tools/explorer/
  
      '''
      fields = fields or ['id', 'owner', 'email', 'name', 'members']
      # TEST that fields are a subset of valid fields
      assert set(fields).issubset(VALID_FIELDS)
  
      get_args = {'fields': ','.join(fields),
                  'access_token': access_token}
      get_args_str = '&'.join(
          ['{}={}'.format(x, y) for x, y in get_args.items()])
  
      base_url = 'https://graph.facebook.com/{}/?{}'.format(
          group_id, get_args_str)
      logr.debug(' LOADING URL: {}'.format(base_url))
  
      response = rq.get(base_url)
  
      # FIXME: check for errors
      # probably want to validate here somehow...?
      r_json = response.json()
      subscribers = r_json.get('members')
  
      return subscribers