Hi again. This quation is related with a previous question.

I'm considering a differential 3-form (mass term), and want to reduce this term from five dimensions down to four.

```
{\Lambda,\Theta,\Sigma,\Xi,\Gamma,\Delta,\Pi,\Omega}::Indices(full,position=independent);
{\mu,\nu,\rho,\sigma,\gamma,\lambda}::Indices(space1,position=independent);
\partial{#}::PartialDerivative;
C_{\Sigma \Pi \Theta}::AntiSymmetric;
third := - a C_{\Sigma \Pi \Theta} C_{\Sigma \Pi \Theta};
split_index(third, $\Sigma,\sigma,4$, repeat=True);
```

At this point I'd like to eliminate (efficiently) the terms such as $C*{4 4 \mu}$ and $C*{444}$.

I expected that `canonicalise()`

would order (i.e. sort) the indices, but it does not

- How can (if possible) sort the indices?
- Is there a way to use the logical
`and`

and`or`

onto the conditional substitution? If so, I could use a single substitution command to kill all irrelevant terms.

Ideas? Thank you!