test :: (GHC.TypeLits.MySet '[Int, Bool, Char] ~ as, GHC.TypeLits.MySet '[Int, Bool, Char] ~ bs) => Proxy as -> Proxy bs
test = id
everything is fine.
I think the problem is that I tried to follow TcTypeNats#typeSymbolCmpTyCon etc style but all the functions there have two inputs and one output with built-in kind (I tried to do [k] kind by myself), so probably I understood something wrong.
Could someone give me a hint what I did wrong, please? I can show my code if needed.