Compute the value of a policy.

        Parameters
        ----------
        sigma : array_like(int, ndim=1)
            Policy vector, of length n.

        Returns
        -------
        v_sigma : ndarray(float, ndim=1)
            Value vector of `sigma`, of length n.

def evaluate_policy(self, sigma):
        """
        Compute the value of a policy.

        Parameters
        ----------
        sigma : array_like(int, ndim=1)
            Policy vector, of length n.

        Returns
        -------
        v_sigma : ndarray(float, ndim=1)
            Value vector of `sigma`, of length n.

        """
        if self.beta == 1:
            raise NotImplementedError(self._error_msg_no_discounting)

        # Solve (I - beta * Q_sigma) v = R_sigma for v
        R_sigma, Q_sigma = self.RQ_sigma(sigma)
        b = R_sigma

        A = self._I - self.beta * Q_sigma

        v_sigma = self._lineq_solve(A, b)

        return v_sigma