def METHOD_NAME(self, index=np.s_[:]):
    """
    @brief 根据散度定理计算多边形的面积
    @note 请注意下面的计算方式不方便实现部分单元面积的计算
    """
    NC = self.number_of_cells()
    node = self.entity('node')
    edge = self.entity('edge')
    edge2cell = self.ds.edge_to_cell()
    t = self.edge_tangent()
    val = t[:, 1]*node[edge[:, 0], 0] - t[:, 0]*node[edge[:, 0], 1] 
    a = np.zeros(NC, dtype=self.ftype)
    np.add.at(a, edge2cell[:, 0], val)
    isInEdge = (edge2cell[:, 0] != edge2cell[:, 1])
    np.add.at(a, edge2cell[isInEdge, 1], -val[isInEdge])
    a /= 2.0
    return a[index]