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

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