@abc.abstractmethod<EOL><INDENT>def M(self, k, t, tips=None, gaps=None):<DEDENT>
pass<EOL>
Matrix exponential `M(mu * t) = exp(mu * t * P)` for category `k`.

        Similar to definition for `Model` abstract base class except
        also specifies for which category `k` we are returning the
        matrix exponential. `k` is an `int` with `0 <= k < ncats`.