docs: def lock_option(self, key, subkey):
          """Make an option unmutable.
  
          :param str key: First identifier of the option.
          :param str subkey: Second identifier of the option.
  
          :raise:
              :NotRegisteredError: If ``key`` or ``subkey`` do not define any
                  option.
          """
          key, subkey = _lower_keys(key, subkey)
          _entry_must_exist(self.gc, key, subkey)
  
          self.gc.loc[
              (self.gc["k1"] == key) &
              (self.gc["k2"] == subkey), "locked"] = True