I have looked through the docs but have not seem what I am looking for, so apologies up front if this is simply me missing something.

My model will run and converge without issues. Due to my specific application, I want to perform algebraic ops on the chains themselves, both in terms of viewing via `arviz`

and more importantly for my workflow. I cannot seem to figure out how to do this though, short of accessing the data in the undrlying `xarray`

objects which seems like too much brute force.

For example, I would like to do something like:

```
with pm.Model():
# stuff
idata = pm.sample(5_000)
# assume `idata` has variables `a` and `b` so that this will
# display the trace for `a` and `b` as expected
az.plot_trace(idata)
# I want to look at the trace for, say `exp(a + b)`
???
```

Thank you very much in advance!