AES: How to create the S-box with Sage The $Sage$ code for the AES $S-box$ is below and a link is here: Polynomial representation of the affine part of the AES S-box. An online version of $Sage$ to test this is here.

I understand most of it but I cannot understand where the polynomials for $a$ to $h$ come from. Can anyone give a simple example? Even for just one of the polynomials if this will suffice.
This might be a bit off-topic, but I can not yet add a comment:

Note that you can also use the SBox object, included in recent Sage versions (from >= 8.2, I think):

sage: from sage.crypto.sboxes import AES
sage: AES
...


This offers you methods for cryptanalysis etc.