The entire reason this `catOptsvProxy__` thing existed was because record updates like `with { sep = 3 }` are potentially type-changing, so GHC did not know what type to infer for `with`.  The solution was to include a dummy field to fix the type `v`.  But now that the `sep` lens is explicitly NOT type-changing this problem goes away, so we can get rid of this silly hack of a field altogether!