SUNFierz[exp, {i, j, k, l}] applies Fierz identity to
the product of two Kronecker deltas the in fundamental representation
(SUNFDelta) carrying indices i,
j, k and l present in
exp.
Overview, SUNSimplify, SUNT, SUNTF, SUNFDelta.
```mathematica SUNFDelta[i, j] SUNFDelta[k, l]
SUNFierz[%, {i, j, k, l}, SUNIndexNames -> {a}]
```mathematica
\delta _{ij} \delta _{kl}
2 T_{il}^a T_{kj}^a+\frac{\delta _{il} \delta _{jk}}{N}