[ITensors] Support for permutation symmetric tensors

[Krzmbrzl]

I have read the ITensors paper and also searched through the online documentation, but I am still not sure whether the ITensors has support for permutational symmetry of indices. For instance, consider the very simple case of $f_{ai} = f_{ia}$: can I tell the package about this symmetry such that it can deduce that it only needs to actually store (and process) a subset of the full $f_{ai}$ matrix?

I've read that the library supports quantum numbers that lead to block-sparse representations and can encode things like rotational symmetry of the underlying problem, but (at least to my understanding) these kinds of symmetries are orthogonal to permutational symmetry of a tensor's indices.

As an example in which this becomes relevant is Coupled Cluster theory (in the context of quantum chemistry) where one has to compute a "residual" that is e.g. $R^{ab}{ij}$ and which has the symmetry $R^{ab}{ij} = R^{ba}_{ji}$ (in the so-called "spin-summed" form).