code: def get_option_alternatives(self, key, subkey):
          
          key, subkey = _lower_keys(key, subkey)
          _entry_must_exist(self.gc, key, subkey)
  
          return self.gc[(self.gc["k1"] == key) &
                         (self.gc["k2"] == subkey)]["values"].values[0]
docs: Get list of available values for an option.
  
          :param str key: First identifier of the option.
          :param str subkey: Second identifier of the option.
  
          :return: Union[:func:`list`, :data:`None`] - alternative values
              for the option, if any specified (otherwise, is open).
  
          :raise:
              :NotRegisteredError: If ``key`` or ``subkey`` do not define any
                  option.