def gets(self, keyword):
        """Return the element of the list after the given keyword as string.

        Parameters
        ----------
        keyword : str
            The keyword parameter to find in the list.
            Putting a colon before the keyword is optional, if no colon is
            given, it is added automatically (e.g. "keyword" will be found as
            ":keyword" in the list).

        Returns
        -------
        obj_str : str
            The string value corresponding to the keyword parameter

        Example:
            kl = KQMLList.from_string('(FAILURE :reason INVALID_PARAMETER)')
            kl.gets('reason') # 'INVALID_PARAMETER'
        """
        param = self.get(keyword)
        if param is not None:
            return safe_decode(param.string_value())
        return None