diff --git a/Symantic/CLI/API.hs b/Symantic/CLI/API.hs
--- a/Symantic/CLI/API.hs
+++ b/Symantic/CLI/API.hs
@@ -124,8 +124,6 @@
 class CLI_Var repr where
 	type VarConstraint repr a :: Constraint
 	var' :: VarConstraint repr a => Name -> repr (a->k) k
-	just :: a -> repr (a->k) k
-	nothing :: repr k k
 	-- Trans defaults
 	type VarConstraint repr a = VarConstraint (UnTrans repr) a
 	default var' ::
@@ -133,17 +131,7 @@
 	 CLI_Var (UnTrans repr) =>
 	 VarConstraint (UnTrans repr) a =>
 	 Name -> repr (a->k) k
-	default just ::
-	 Trans repr =>
-	 CLI_Var (UnTrans repr) =>
-	 a -> repr (a->k) k
-	default nothing ::
-	 Trans repr =>
-	 CLI_Var (UnTrans repr) =>
-	 repr k k
-	var'    = noTrans . var'
-	just    = noTrans . just
-	nothing = noTrans nothing
+	var' = noTrans . var'
 
 -- | Like 'var'' but with the type variable @(a)@ first instead or @(repr)@
 -- so it can be passed using 'TypeApplications' without adding a |\@_| for @(repr)@.
@@ -155,6 +143,27 @@
 var = var'
 {-# INLINE var #-}
 
+-- * Class 'CLI_Var'
+class CLI_Constant repr where
+	constant :: Segment -> a -> repr (a->k) k
+	just     :: a -> repr (a->k) k
+	nothing  :: repr k k
+	default constant ::
+	 Trans repr =>
+	 CLI_Constant (UnTrans repr) =>
+	 Segment -> a -> repr (a->k) k
+	default just ::
+	 Trans repr =>
+	 CLI_Constant (UnTrans repr) =>
+	 a -> repr (a->k) k
+	default nothing ::
+	 Trans repr =>
+	 CLI_Constant (UnTrans repr) =>
+	 repr k k
+	constant s = noTrans . constant s
+	just       = noTrans . just
+	nothing    = noTrans nothing
+
 -- * Class 'CLI_Env'
 class CLI_Env repr where
 	type EnvConstraint repr a :: Constraint
@@ -191,7 +200,7 @@
 	 cs -> TagLong cs
 
 -- * Class 'CLI_Tag'
-class (App repr, Permutable repr, CLI_Var repr) => CLI_Tag repr where
+class (App repr, Permutable repr, CLI_Constant repr) => CLI_Tag repr where
 	type TagConstraint repr a :: Constraint
 	tag :: Tag -> repr f k -> repr f k
 	-- tag n = (tag n <.>)
diff --git a/Symantic/CLI/Help.hs b/Symantic/CLI/Help.hs
--- a/Symantic/CLI/Help.hs
+++ b/Symantic/CLI/Help.hs
@@ -169,8 +169,10 @@
 instance SchemaDoc d => CLI_Var (Help d) where
 	type VarConstraint (Help d) a = ()
 	var' n  = Help mempty (var' n)
-	just a  = Help mempty (just a)
-	nothing = Help mempty nothing
+instance SchemaDoc d => CLI_Constant (Help d) where
+	constant n a = Help mempty (constant n a)
+	just a       = Help mempty (just a)
+	nothing      = Help mempty nothing
 instance SchemaDoc d => CLI_Env (Help d) where
 	type EnvConstraint (Help d) a = ()
 	env' n =
diff --git a/Symantic/CLI/Layout.hs b/Symantic/CLI/Layout.hs
--- a/Symantic/CLI/Layout.hs
+++ b/Symantic/CLI/Layout.hs
@@ -272,6 +272,14 @@
 			where h | List.null (layoutInh_message inh) = []
 			        | otherwise = [(docSchema sch, layoutInh_message inh)]
 		where sch = var' n
+instance LayoutDoc d => CLI_Constant (Layout d) where
+	constant c a = Layout sch [] $ \inh -> do
+		modify' $ \k -> \case
+		 Nothing -> k Nothing
+		 Just ts -> k $ Just [Tree.Node (LayoutNode_List [] h) ts]
+			where h | List.null (layoutInh_message inh) = []
+			        | otherwise = [(docSchema sch, layoutInh_message inh)]
+		where sch = constant c a
 	just a  = Layout (just a) [] $ \_inh -> pure ()
 	nothing = Layout nothing  [] $ \_inh -> pure ()
 instance LayoutDoc d => CLI_Env (Layout d) where
diff --git a/Symantic/CLI/Parser.hs b/Symantic/CLI/Parser.hs
--- a/Symantic/CLI/Parser.hs
+++ b/Symantic/CLI/Parser.hs
@@ -42,6 +42,7 @@
 import qualified Data.Map.Merge.Strict as Map
 import qualified Data.Map.Strict as Map
 import qualified Data.Set as Set
+import qualified System.Exit as System
 import qualified Data.Text as Text
 import qualified Data.Text.IO as Text
 import qualified Data.Text.Lazy as TL
@@ -73,6 +74,7 @@
 				"Error parsing the command at argument #" <>
 				show (P.errorOffset e + 1) <> ":\n" <>
 				parseErrorTextPretty e
+			System.exitWith (System.ExitFailure 2)
 	 Right app -> unResponseParser $ app handlers
 
 -- | Rewrite 'P.parseErrorTextPretty' to keep 'Ord' of 'Arg'.
@@ -205,6 +207,9 @@
 		 Right a -> return ($ a)
 		where
 		expName = Set.singleton $ P.Label $ NonEmpty.fromList $ "<"<>name<>">"
+instance Ord e => CLI_Constant (Parser e d) where
+	constant "" a = just a
+	constant c  a = commands Map.empty (Map.singleton c (just a))
 	just a  = Parser $ return ($ a)
 	nothing = Parser $ return id
 instance Ord e => CLI_Env (Parser e d) where
@@ -322,7 +327,7 @@
  , Reflection.Typeable a
  ) => Outputable (Maybe a) where
 	output = \case
-	 Nothing -> return ()
+	 Nothing -> System.exitWith (System.ExitFailure 1)
 	 Just a  -> output a
 instance
  ( Reflection.Typeable e
@@ -331,7 +336,9 @@
  , Outputable a
  ) => Outputable (Either e a) where
 	output = \case
-	 Left e -> output $ OnHandle IO.stderr e
+	 Left e -> do
+		output (OnHandle IO.stderr e)
+		System.exitWith (System.ExitFailure 1)
 	 Right a -> output a
 
 -- * Class 'IOType'
@@ -528,6 +535,7 @@
 		 (Map.fromAscList $ (,x) <$> preCmds)
 		 (Map.fromAscList $ (,x) <$> cmds)
 instance Ord e => CLI_Var (Router (Parser e d))
+instance Ord e => CLI_Constant (Router (Parser e d))
 instance Ord e => CLI_Env (Router (Parser e d))
 instance Ord e => CLI_Tag (Router (Parser e d)) where
 	tag = Router_Tag
diff --git a/Symantic/CLI/Schema.hs b/Symantic/CLI/Schema.hs
--- a/Symantic/CLI/Schema.hs
+++ b/Symantic/CLI/Schema.hs
@@ -58,7 +58,7 @@
  =   SchemaInh
  {   schemaInh_op     :: (Infix, Side) -- ^ Parent operator.
  ,   schemaInh_define :: Bool          -- ^ Whether to print a definition, or not.
- ,   schemaInh_or     :: d
+ ,   schemaInh_or     :: d             -- ^ The separator to use between alternatives.
  }
 
 defSchemaInh :: SchemaDoc d => SchemaInh d
@@ -228,6 +228,9 @@
 	type VarConstraint (Schema d) a = ()
 	var' n = Schema $ \_inh -> Just $
 		Doc.underline $ Doc.from $ Doc.Word n
+instance SchemaDoc d => CLI_Constant (Schema d) where
+	constant c _a = Schema $ \inh -> Just $
+		Doc.from (Doc.Word c)
 	just _  = Schema $ \_inh -> Nothing
 	nothing = Schema $ \_inh -> Nothing
 instance SchemaDoc d => CLI_Env (Schema d) where
diff --git a/symantic-cli.cabal b/symantic-cli.cabal
--- a/symantic-cli.cabal
+++ b/symantic-cli.cabal
@@ -2,7 +2,7 @@
 -- PVP:  +-+------- breaking API changes
 --       | | +----- non-breaking API additions
 --       | | | +--- code changes with no API change
-version: 2.4.0.20190719
+version: 2.4.2.20190806
 category: System, CLI, Options, Parsing
 synopsis: Symantics for parsing and documenting a CLI
 description: