```
{a,b,c,d}::Indices(full,position=fixed).
substitute($S_{b}$, $S_{a} -> T^{b}U_{b}P_{a}$);
```

complains: `cadabra2.ConsistencyException: Fixed index pair with two upper or two lower indices found`

. Is this a bug? If not, how should I proceed?

(What I'm actually trying to achieve, to understand Cadabra better, is to write the Riemann tensor in terms of the metric. Of course I do this by writing the Riemann tensor in terms of Christoffel symbols then substituting their expression in terms of the metric. In that second step I get the same `ConsistencyException`

as my toy model above.)