I have to compute:

`H^{a b} \nabla_{m}{\nabla_{n}{H_{r s}}} \commutator{ \commutator{\Gamma^{m}}{\Gamma_{a b}} }{\Gamma^{n r s}};`

I expand the commutators:

```
@substitute!(%)(\commutator{ \commutator{\Gamma^{m}}{\Gamma_{a b}} }{\Gamma^{n r s}} -> \commutator{\Gamma^{m}}{\Gamma_{a b}} \Gamma^{n r s} - \Gamma^{n r s} \commutator{\Gamma^{m}}{\Gamma_{a b}} );
@substitute!(%)( \commutator{\Gamma^{m}}{\Gamma_{a b}} -> \Gamma^{m} \Gamma_{a b} - \Gamma_{a b} \Gamma^{m} );
```

and expand out gamma products

```
@distribute!(%);
@join!(%){expand};
@distribute!(%);
@join!(%){expand};
@distribute!(%);
```

then I want to put `\gamma_{mn} = 0`

(cause I am computing the trace of this object)

```
@substitute!(%)( \Gamma^{m n} -> 0 );
@rename_dummies!(%);
```

but the result is not = 0 as it should be, the subsitution is partial only apparently!