Proving equality of classes is unexplained

[sxhya]

To be able show that the StateMonad is a monad, you need to be able to prove equality of two instances of classes. Even I only know that this can be done using tactic ext and don't how this can be done in Arend w/o tactics. Add more hints to the exercise.

[valis]

It can be done quite easily without tactics or anything that was not explained at this point.