Cannot calculate matrix inverse of mixed tensors

[eschnett]

It is not possible to calculate the inverse of J[ua,lb]; Kranc expects all indices to be lower when calling MatrixInverse.

A work-around is to define a shorthand with all lower indices and to contract with KD to "lower" the index.

[ianhinder]

Yes. The fix would be to extend MatrixInverse to handle the cases that it does not currently handle. I've never been happy with MatrixInverse, as it is a bit confusing. We should think of a better way of computing the inverse of a matrix in a way which maintains the index positions and doesn't look "wrong".