Hi,

I have expressions like these

```
\gamma_{\alpha \gamma} \gamma_{\beta \delta} \gamma^{\epsilon \zeta} \partial_{\epsilon \zeta}(U^{\alpha \beta}) dU^{\gamma \delta} +
\gamma_{\alpha \gamma} \gamma^{\delta \epsilon} \gamma_{\beta \zeta} \partial_{\delta \epsilon}(U^{\alpha \beta}) dU^{\gamma \zeta} +
\gamma^{\gamma \delta} \gamma_{\alpha \epsilon} \gamma_{\beta \zeta} \partial_{\gamma \delta}(U^{\alpha \beta}) dU^{\epsilon \zeta}
```

Which don't simplify properly, which is because the summands don't canonicalize properly. It would be solved by sorting `\gamma^{..}`

before `\gamma_{..}`

(or vice versa) and then relabelling the indices. I just can't figure out how to do this. Can I declare a SortOrder for this?

(Please feel free to edit my question such that the formulas are displayed with LaTeX, I couldn't figure out how to do that.)