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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 compute_accumulated_cost_matrix(self):
 """ Compute the accumulated cost matrix, and return it. Return ``None`` if the accumulated cost matrix cannot be computed because one of the two waves is empty after masking (if requested). :rtype: :class:`numpy.ndarray` (2D) :raises: RuntimeError: if both the C extension and the pure Python code did not succeed. .. versionadded:: 1.2.0 """

        self._setup_dtw()
        if self.dtw is None:
            self.log(u"Inner self.dtw is None => returning None")
            return None
        self.log(u"Returning accumulated cost matrix")
        return self.dtw.compute_accumulated_cost_matrix()