instance {-# OVERLAPPING #-} (Validatable' r, KnownSymbol s) => Validatable' (MetaX s r) where
  form' = M1 <$> (fieldName DIG..: form')
    where
      fieldName = pack $ symbolVal (Proxy :: Proxy s)

instance {-# OVERLAPPABLE #-} (Validatable' r) => Validatable' (M1 i a r) where
  form' = M1 <$> form'

instance (Validatable' r, Validatable' s) => Validatable' (r :*: s) where
  form' = (:*:) <$> form' <*> form'

-- all other instances as before