type family Nub t where ...
type family Sort (xs :: [k]) :: [k] where ...
type family (m :: [k]) \\ (c :: Symbol) :: [k] where ...
type family Lookup (m :: [k']) (c :: k) :: Maybe v where ...
type family Lookup' (m :: [k']) (c :: k) :: v where ...
type family Combine (a :: v) (b :: v) :: v