docs: def get_option_type(self, key, subkey):
          """Get the type of a particular option.
  
          :param str key: First identifier of the option.
          :param str subkey: Second identifier of the option.
  
          :return: :class:`str` - description of the type.
  
          :raise:
              :NotRegisteredError: If ``key`` or ``subkey`` do not define any
                  option.
          """
          key, subkey = _lower_keys(key, subkey)
          _entry_must_exist(self.gc, key, subkey)
  
          return self.gc[(self.gc["k1"] == key) &
                         (self.gc["k2"] == subkey)]["type"].values[0]