code: def reset_option(self, key, subkey):
          
          if not self.open:
              return
  
          key, subkey = _lower_keys(key, subkey)
          _entry_must_exist(self.gc, key, subkey)
  
          df = self.gc[(self.gc["k1"] == key) & (self.gc["k2"] == subkey)]
          if df["locked"].values[0]:
              raise ValueError("{0}.{1} option is locked".format(key, subkey))
          val = df["default"].values[0]
          self.gc.loc[
              (self.gc["k1"] == key) &
              (self.gc["k2"] == subkey), "value"] = val
docs: Resets a single option to the default values.
  
          :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.
              :ValueError: If the targeted obtion is locked.