when I use

```
{\mu,\nu}::Indices(vector).
tr{#}::Trace.
ex:=A A tr{u^{\mu} u^{\nu}} tr{u^{\nu} u^{\mu}}- B B tr{u^{\mu} u^{\nu}} tr{u^{\mu} u^{\nu}};
sort_product(_);
collect_factors(_);
factor_in(_,$A**2,B**2$);
```

I get the following error

{\color{red}{\begin{verbatim}RuntimeError: Power with free indices not allowed.

At: (4): \end{verbatim}}}

How to solve it?