@abc.abstractmethod<EOL><INDENT>def stationarystate(self, k):<DEDENT>
pass<EOL>
Stationary state of substitution model.

        Args:
            `k` (int)
                Category for which we are computing stationary state.

        A `numpy.ndarray` of floats, shape `(k, nsites, N_CODON)`.
        Element `stationarystate[k][r][x]` is the stationary
        state probability of codon `x` at site `r` for category `k`.