@abc.abstractmethod<EOL><INDENT>def dM(self, k, t, param, Mkt, tips=None, gaps=None):<DEDENT>
pass<EOL>
Derivative of `M(k, t)` with respect to `param`.

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

        Also, `param` can be any string in `freeparams` **except**
        those in `distributionparams`, and can also be `distributedparam`.