A_{i_{\ell+1} j_{\ell+1}}^{(\ell+1)} = \mathcal F\left( \left\{ A_{i_\ell j_\ell}^{(\ell)} \right\}_{i_\ell \in i_{\ell+1}, j_\ell \in j_{\ell+1}} \right) | A_{i_{\ell+1} j_{\ell+1}}^{(\ell+1)} = \mathcal F\left( \left\{ A_{i_\ell j_\ell}^{(\ell)} \right\}_{i_\ell \in i_{\ell+1}, j_\ell \in j_{\ell+1}} \right) |