For example,

```
{\mu,\nu}::Indices(vector).
{a^{\mu},b^{\mu}}::NonCommuting.
ex:=a b a^{\mu} a b_{\mu} b+a b b^{\mu} b a_{\mu} a;
canonicalise(_);
```

I want to move 'a' & 'b' to left side, but I don't know how to do it ('factor_out' is not simple enough to do it).