The following code

```
{\mu,\nu,\rho,\sigma}::Indices(vector).
\epsilon^{\mu\nu\rho\sigma}::EpsilonTensor.
{a^{\mu},b^{\mu}}::NonCommuting.
{a^{\mu},b^{\mu}}::SelfNonCommuting.
tr{#}::Trace.
ts:=\epsilon^{\mu\nu\rho\sigma}\tr{a^{\mu}a^{\nu}b^{\rho}b^{\sigma}}+\epsilon^{\mu\nu\rho\sigma}\tr{a^{\mu}b^{\nu}b^{\rho}a^{\sigma}};
meld(_);
sort_product(_);
canonicalise(_);
```

can't get the right result, are there some bugs here?