-- >>> :t test
-- test :: Proxy '["a", "b"] -> Proxy '["a", "b"]
test :: (xs ~ ("a" ': "b" ': rest), DeepSeq xs) => Proxy xs -> Proxy xs
test = id

-- >>> :t xsHasA
-- xsHasA :: Proxy ("a" : xs')
xsHasA :: "a" `Elem` xs => Proxy xs
xsHasA = Proxy

-- >>> :t xsHasB
-- xsHasB :: Proxy ("b" : xs')
xsHasB :: "b" `Elem` xs => Proxy xs
xsHasB = Proxy

-- >>> :t automaticallyReorderTypeVars
-- automaticallyReorderTypeVars :: [Proxy ("a" : "b" : xs')]
automaticallyReorderTypeVars = [xsHasA, xsHasB]

class (x :: a) `Elem` (xs :: [a])
type family x `ElemMatch` xs :: Constraint where
    x `ElemMatch` (x ': xs) = ()
    x `ElemMatch` (_ ': xs) = x `Elem` xs
instance (TrySpecializeAs (x ': xs') xs, x `ElemMatch` xs) => x `Elem` xs