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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _M(self, k, t, tips=None, gaps=None):
 """Returns matrix exponential `M`."""

        if self._distributionmodel:
            return self.model.M(k, t, tips, gaps)
        else:
            return self.model.M(t, tips, gaps)