Dear Cadabra developers and users,

I would like to construct a "scalar product" of tensors of the form:

met:X x Y ->X*{a}*Y*{a}

but I can't find a way to implement it.

When acting on scalar quantities, it works fine as follows:

```
{a, b, c, d, e, f, g, h, i, j, k, l, m, n ,p, q, r,s,t,u,v,w,x,y,z#}::Indices(T, position=free, parent=double);
Expmet:=met{X??}{Y??}->X??*Y??;
Exp:=met{V}{W};
substitute(_, Expmet);
```

and the output is `VW`

as expected.

Now, if I want X and Y to be vectors, then the naive guess

`Expmet:=met{X??}{Y??}->{X??}_{a}*{Y??}_{a};`

doesn't work, the output being (VW)_{aa}.

I tried several variations but none of them worked so far.

Is there a way to implement this kind of operators for tensors?

Thank you already for any input!