How do I define a non-trivial tensor symmetry?

[Daigaku no Baku]

Hello everyone! I apologise in advance if my question is stupid or irrelevant, but I can't find an answer for it for three days already. I hope I don't break some rule writing this.
I am trying to define a sympy tensor.tensor.TensorSymmetry for a tensor A^i_\mu\nu ( A(i, -mu, -nu)) that has three indices, i, mu, nu. Indices mu and nu live in the same bundle, i lives on a different bundle. The tensor is skew-symmetric with respect to (mu, nu) but has no symmetry with respect to (mu, i) or (nu, i). I struggle to understand how do I define such a tensor symmetry. The documentation for sympy.combinatorics.tensor_can is very opaque and I can't understand what is going on there, additionally, I am not familiar with the BSGS formalism, and it's hard to find a good explanation of it, too. Is there a guide or something, or perhaps someone could give an example? Thank you in advance!