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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 rate(self, pores=[], throats=[], mode='group'):
 r""" Calculates the net rate of material moving into a given set of pores or throats Parameters pores : array_like The pores for which the rate should be calculated throats : array_like The throats through which the rate should be calculated mode : string, optional Controls how to return the rate. Options are: *'group'*: (default) Returns the cumulative rate of material moving into the given set of pores *'single'* : Calculates the rate for each pore individually Returns ------- If ``pores`` are specified, then the returned values indicate the net rate of material exiting the pore or pores. Thus a positive rate indicates material is leaving the pores, and negative values mean material is entering. If ``throats`` are specified the rate is calculated in the direction of the gradient, thus is always positive. If ``mode`` is 'single' then the cumulative rate through the given pores (or throats) are returned as a vector, if ``mode`` is 'group' then the individual rates are summed and returned as a scalar. """

        pores = self._parse_indices(pores)
        throats = self._parse_indices(throats)

        network = self.project.network
        phase = self.project.phases()[self.settings['phase']]
        g = phase[self.settings['conductance']]
        quantity = self[self.settings['quantity']]

        P12 = network['throat.conns']
        X12 = quantity[P12]
        f = (-1)**np.argsort(X12, axis=1)[:, 1]
        Dx = np.abs(np.diff(X12, axis=1).squeeze())
        Qt = -f*g*Dx

        if len(throats) and len(pores):
            raise Exception('Must specify either pores or throats, not both')
        elif len(throats):
            R = np.absolute(Qt[throats])
            if mode == 'group':
                R = np.sum(R)
        elif len(pores):
            Qp = np.zeros((self.Np, ))
            np.add.at(Qp, P12[:, 0], -Qt)
            np.add.at(Qp, P12[:, 1], Qt)
            R = Qp[pores]
            if mode == 'group':
                R = np.sum(R)
        return np.array(R, ndmin=1)