code: def unregister_option(self, key, subkey):
          
          if not self.open:
              return
  
          key, subkey = _lower_keys(key, subkey)
          _entry_must_exist(self.gc, key, subkey)
  
          self.gc = self.gc[~((self.gc['k1'] == key) &
                              (self.gc['k2'] == subkey))]
docs: Removes an option from the manager.
  
          :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.