@abc.abstractmethod<EOL><INDENT>def dstationarystate(self, k, param):<DEDENT>
pass<EOL>
Derivative of `stationarystate` with respect to `param` for category `k`.

        Args:
            `k` (int)
                Category for which we are computing stationary state.
            `param` (string)
                A string in `freeparams` or `distributedparam`

        Returns:
            `dstationarystate` (`numpy.ndarray` of floats or zero)
                If `param` is a float, then `dstationarystate[k][r][x]`
                is derivative of `stationarystate[k][r][x]` with respect
                to `param` for category `k`. If `param` is an array, then
                `dstationarystate[k][i][r][x]` is derivative of
                `stationarystate[k][r][x]` with respect to `param[i]` for
                category `k`.