def _dM(self, k, t, param, M, tips=None, gaps=None):
if self._distributionmodel:<EOL><INDENT>return self.model.dM(k, t, param, M, tips, gaps)<EOL><DEDENT>else:<EOL><INDENT>return self.model.dM(t, param, M, tips, gaps)<EOL><DEDENT>
Returns derivative of matrix exponential.