diff --git a/haskus-utils-types.cabal b/haskus-utils-types.cabal
--- a/haskus-utils-types.cabal
+++ b/haskus-utils-types.cabal
@@ -1,29 +1,35 @@
-cabal-version: >=1.20
-name: haskus-utils-types
-version: 1.2
-license: BSD3
-license-file: LICENSE
-copyright: Sylvain Henry 2018
-maintainer: sylvain@haskus.fr
-author: Sylvain Henry
-homepage: http://www.haskus.org
-synopsis: Haskus utility modules
+name:                haskus-utils-types
+version:             1.3
+synopsis:            Haskus utility modules
+license:             BSD3
+license-file:        LICENSE
+author:              Sylvain Henry
+maintainer:          sylvain@haskus.fr
+homepage:            http://www.haskus.org
+copyright:           Sylvain Henry 2018
+category:            System
+build-type:          Simple
+cabal-version:       1.20
+
 description:
-    Haskus types utility modules
-category: System
-build-type: Simple
+   Haskus types utility modules
 
 source-repository head
-    type: git
-    location: git://github.com/haskus/haskus-utils.git
+  type: git
+  location: git://github.com/haskus/haskus-utils.git
 
 library
-    exposed-modules:
-        Haskus.Utils.Types
-        Haskus.Utils.Types.List
-        Haskus.Utils.Types.Generics
-    hs-source-dirs: src/lib
-    default-language: Haskell2010
-    ghc-options: -Wall
-    build-depends:
-        base >=4.9 && <4.12
+  exposed-modules:
+    Haskus.Utils.Types
+    Haskus.Utils.Types.List
+    Haskus.Utils.Types.Generics
+
+  other-modules:
+
+  build-depends:       
+         base                      >= 4.9 && < 5
+
+  build-tools: 
+  ghc-options:          -Wall
+  default-language:     Haskell2010
+  hs-source-dirs:       src/lib
diff --git a/src/lib/Haskus/Utils/Types/List.hs b/src/lib/Haskus/Utils/Types/List.hs
--- a/src/lib/Haskus/Utils/Types/List.hs
+++ b/src/lib/Haskus/Utils/Types/List.hs
@@ -36,13 +36,13 @@
    , ReplaceN
    , ReplaceNS
    -- * Set operations
-   , IsMember
-   , IsSubset
+   , CheckMember
+   , CheckMembers
    , Union
    , Complement
    , Product
    , Member
-   , Member'
+   , Members
    , CheckNub
    -- * Index operations
    , IndexOf
@@ -62,6 +62,7 @@
 where
 
 import Haskus.Utils.Types
+import GHC.Exts (Constraint)
 
 -- | Map a type function
 type family Map (f :: a -> k) (xs :: [a]) :: [k] where
@@ -184,33 +185,34 @@
    Generate n m = n ': Generate (n+1) m
 
 -- | Check that a type is member of a type list
-type family IsMember (a :: k) (l :: [k]) :: Bool where
-   IsMember a l = IsMember' l a l
+type family CheckMember (a :: k) (l :: [k]) :: Constraint where
+   CheckMember a l = CheckMember' l a l
 
--- | Helper for IsMember
-type family IsMember' (i :: [k]) (a :: k) (l :: [k]) :: Bool where
-   IsMember' i a (a ': l) = 'True
-   IsMember' i a (b ': l) = IsMember' i a l
-   IsMember' i a '[]      = TypeError ( 'Text "`"
-                                   ':<>: 'ShowType a
-                                   ':<>: 'Text "'"
-                                   ':<>: 'Text " is not a member of "
-                                   ':<>: 'ShowType i)
+-- | Helper for CheckMember
+type family CheckMember' (i :: [k]) (a :: k) (l :: [k]) :: Constraint where
+   CheckMember' i a (a ': l) = ()
+   CheckMember' i a (b ': l) = CheckMember' i a l
+   CheckMember' i a '[]      = TypeError ( 'Text "`"
+                                     ':<>: 'ShowType a
+                                     ':<>: 'Text "'"
+                                     ':<>: 'Text " is not a member of "
+                                     ':<>: 'ShowType i)
 
 
 -- | Check that a list is a subset of another
-type family IsSubset (l1 :: [k]) (l2 :: [k]) :: Bool where
-   IsSubset l1 l1 = 'True
-   IsSubset l1 l2 = IsSubset' l2 l1 l2
+type family CheckMembers (l1 :: [k]) (l2 :: [k]) :: Constraint where
+   CheckMembers l1 l1 = ()
+   CheckMembers l1 l2 = CheckMembers' l2 '[] l1 l2
 
--- | Helper for IsSubset
-type family IsSubset' (i :: [k]) (l1 :: [k]) (l2 :: [k]) :: Bool where
-   IsSubset' i '[] l2 = 'True
-   IsSubset' i l1 '[] = TypeError (     'ShowType l1
-                                   ':$$: 'Text "is not a subset of"
-                                   ':$$: 'ShowType i)
-   IsSubset' i (x ': xs) (x ': ys) = IsSubset' i xs i
-   IsSubset' i (x ': xs) (y ': ys) = IsSubset' i (x ': xs) ys
+-- | Helper for CheckMembers
+type family CheckMembers' (i :: [k]) (e :: [k]) (l1 :: [k]) (l2 :: [k]) :: Constraint where
+   CheckMembers' i e '[] l2  = ()
+   CheckMembers' i e '[] '[] = TypeError (   'ShowType e
+                                       ':$$: 'Text "is not a subset of"
+                                       ':$$: 'ShowType i)
+   CheckMembers' i e (l ': ls) '[]       = CheckMembers' i (l ': e) ls i
+   CheckMembers' i e (x ': xs) (x ': ys) = CheckMembers' i e xs i
+   CheckMembers' i e (x ': xs) (y ': ys) = CheckMembers' i e (x ': xs) ys
 
 -- | Get list indexes
 type family Indexes (l :: [k]) :: [Nat] where
@@ -310,16 +312,14 @@
 
 -- | Constraint: x member of xs
 type Member x xs =
-   ( IsMember x xs ~ 'True
-   , x ~ Index (IndexOf x xs) xs
-   , KnownNat (IndexOf x xs)
-   )
-
--- | Constraint: x member of xs (silent)
-type Member' x xs =
    ( x ~ Index (IndexOf x xs) xs
    , KnownNat (IndexOf x xs)
    )
+
+-- | Constraint: all the xs are members of ys
+type family Members xs ys :: Constraint where
+   Members '[] ys       = ()
+   Members (x ': xs) ys = (Member x ys, Members xs ys)
 
 -- | Check that a list only contain a value of each type
 type CheckNub (l :: [k]) =