A newbie question, I guess. I am challenged to zoom in on specific terms and apply a rule to them.

I want to say "zoom in on the 4th and 8th term in a 9 term statement and apply a substitution rule", **specifically when all the terms** are complex expressions with multiple indexes and partial derivatives. How do I do that?

In more detail, I have a complex expression with 9 terms and each term has a lot of indexes, but for simplicity lets just call these 9 terms

A + B + C + D + E + F + G + H + I

What I want to do, for example, is zoom in on terms D and H and apply a subsitution rule which is itself involving a lot of indexes and a partial derivative.

My problem is that all these terms involve partial derivatives as well as indexes and zoom does not seem to capture these well.

Here is example code that provides perhaps some context:

```
from cdb.relativity.abstract import *
from cdb.core.manip import *
g_{\mu\nu}::Metric.
g_{\mu\nu}::Depends(\partial{#}).
g^{\mu\nu}::InverseMetric.
g^{\mu\nu}::Depends(\partial{#}).
\Gamma^{\mu}_{\nu\rho}::Depends(\partial{#}).
ch = christoffel_from_metric();
ch_2 := \Gamma^{\rho}_{\nu\sigma}\Gamma^{\sigma}_{\rho\mu};
substitute(ch_2,ch);
distribute(_)
sort_product(_)
sort_sum(_)
rename_dummies(_);
```

Now how do I `zoom`

in on the 4th and 8th terms of this last expression? It can be through the `zoom()`

function, but I am also happy with `take_match()`

or any of the `manip`

functions. I wasn't able to solve it with them.

Many thanks GPN