type family (x :: [k]) ++ (y :: [k]) :: [k] where ...
type family Nub t where ...
type family Sort (xs :: [k]) :: [k] where ...
type family Filter (f :: Bool) (p :: k) (xs :: [k]) :: [k] where ...
type family IfCons (pred :: Bool) (x :: k) (xs :: [k]) :: [k] where ...