diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -1,4 +1,10 @@
-# Unreleased
+# 0.7.0.0
+
+* Write trace messages to stderr.
+
+# 0.6.0.0
+
+* Add polymorphic type family `Undefined` that emits a `TypeError`.
 
 # 0.5.0.0
 
diff --git a/incipit-base.cabal b/incipit-base.cabal
--- a/incipit-base.cabal
+++ b/incipit-base.cabal
@@ -1,11 +1,11 @@
 cabal-version: 2.2
 
--- This file has been generated from package.yaml by hpack version 0.35.0.
+-- This file has been generated from package.yaml by hpack version 0.38.3.
 --
 -- see: https://github.com/sol/hpack
 
 name:           incipit-base
-version:        0.5.1.0
+version:        0.7.0.2
 synopsis:       A Prelude for Polysemy – Base Reexports
 description:    See https://hackage.haskell.org/package/incipit-base/docs/IncipitBase.html
 category:       Prelude
@@ -13,7 +13,7 @@
 bug-reports:    https://github.com/tek/incipit-core/issues
 author:         Torsten Schmits
 maintainer:     hackage@tryp.io
-copyright:      2023 Torsten Schmits
+copyright:      2025 Torsten Schmits
 license:        BSD-2-Clause-Patent
 license-file:   LICENSE
 build-type:     Simple
@@ -75,6 +75,7 @@
   hs-source-dirs:
       lib
   default-extensions:
+      NoImplicitPrelude
       AllowAmbiguousTypes
       ApplicativeDo
       BlockArguments
@@ -89,9 +90,12 @@
       GADTs
       LambdaCase
       LiberalTypeSynonyms
+      MonadComprehensions
       MultiWayIf
+      NoFieldSelectors
       OverloadedLabels
       OverloadedLists
+      OverloadedRecordDot
       OverloadedStrings
       PackageImports
       PartialTypeSignatures
@@ -107,42 +111,12 @@
       UndecidableInstances
       UnicodeSyntax
       ViewPatterns
-      BangPatterns
-      BinaryLiterals
-      ConstraintKinds
-      DeriveDataTypeable
-      DeriveFoldable
-      DeriveFunctor
-      DeriveGeneric
-      DeriveLift
-      DeriveTraversable
-      DoAndIfThenElse
-      EmptyCase
-      EmptyDataDecls
-      ExistentialQuantification
-      FlexibleContexts
-      FlexibleInstances
-      GeneralizedNewtypeDeriving
-      InstanceSigs
-      KindSignatures
-      MultiParamTypeClasses
-      NamedFieldPuns
-      PatternGuards
-      PolyKinds
-      RankNTypes
-      ScopedTypeVariables
-      StandaloneDeriving
-      TupleSections
-      TypeApplications
-      TypeOperators
-      TypeSynonymInstances
-      NoImplicitPrelude
   ghc-options: -Wall -Widentities -Wincomplete-uni-patterns -Wmissing-deriving-strategies -Wredundant-constraints -Wunused-type-patterns -Wunused-packages
   build-depends:
-      base >=4.13 && <4.19
-    , bytestring
-    , containers
-    , data-default ==0.7.*
-    , stm
-    , text
-  default-language: Haskell2010
+      base >=4.17.2.1 && <4.23
+    , bytestring >=0.11.1.0 && <0.13
+    , containers >=0.6.3.1 && <0.9
+    , data-default >=0.2 && <0.9
+    , stm >=2.5.1.0 && <2.6
+    , text >=2.0.2 && <2.2
+  default-language: GHC2021
diff --git a/lib/Incipit/Base.hs b/lib/Incipit/Base.hs
--- a/lib/Incipit/Base.hs
+++ b/lib/Incipit/Base.hs
@@ -1,3 +1,5 @@
+{-# language CPP #-}
+
 -- |Reexports from @base@.
 module Incipit.Base (
   module Control.Applicative,
@@ -32,6 +34,9 @@
   module Data.Traversable,
   module Data.Tuple,
   module Data.Typeable,
+#if __GLASGOW_HASKELL__ >= 904
+  module Data.Type.Equality,
+#endif
   module Data.Void,
   module Data.Word,
   module GHC.Base,
@@ -156,6 +161,9 @@
 import Data.Semigroup (Semigroup (..))
 import Data.Traversable (Traversable (..), for, forM, mapAccumL, mapAccumR)
 import Data.Tuple (curry, fst, snd, swap, uncurry)
+#if __GLASGOW_HASKELL__ >= 904
+import Data.Type.Equality (type (~))
+#endif
 import Data.Typeable (Typeable)
 import Data.Void (Void)
 import Data.Word (Word, Word16, Word32, Word64, Word8)
diff --git a/lib/Incipit/Debug.hs b/lib/Incipit/Debug.hs
--- a/lib/Incipit/Debug.hs
+++ b/lib/Incipit/Debug.hs
@@ -6,19 +6,18 @@
 import qualified Data.Text as Text
 import Data.Text (Text)
 import GHC.Stack (CallStack, SrcLoc (..), callStack, getCallStack)
+import System.IO (IO, hPutStrLn, stderr)
 import System.IO.Unsafe (unsafePerformIO)
 
 import Incipit.Base (
   Applicative (pure),
   Functor ((<$)),
   HasCallStack,
-  IO,
   Monad,
   Semigroup ((<>)),
   Show,
   error,
   fromMaybe,
-  putStrLn,
   )
 import Incipit.List (last)
 import Incipit.String.Conversion (ToString (toString), ToText (toText), show)
@@ -33,7 +32,7 @@
   Text ->
   IO ()
 debugPrint SrcLoc {srcLocModule = (toText -> slm), srcLocStartLine} !msg =
-  putStrLn (toString moduleName <> ":" <> show srcLocStartLine <> " " <> toString msg)
+  hPutStrLn stderr (toString moduleName <> ":" <> show srcLocStartLine <> " " <> toString msg)
   where
     moduleName =
       fromMaybe slm (last (Text.splitOn "." slm))
@@ -68,7 +67,7 @@
   m ()
 dbgs a =
   debugPrintWithLoc (srcLoc callStack) (show a)
-{-# noinline dbgs_ #-}
+{-# noinline dbgs #-}
 
 -- |Print a value with a 'Show' instance in an arbitrary 'Monad', returning the value.
 dbgs_ ::
@@ -80,7 +79,7 @@
   m a
 dbgs_ a =
   a <$ debugPrintWithLoc (srcLoc callStack) (show a)
-{-# noinline dbgs #-}
+{-# noinline dbgs_ #-}
 
 -- |Like 'Debug.Trace.trace', but with 'Text' and with source location prefix.
 tr ::
diff --git a/lib/Incipit/Foldable.hs b/lib/Incipit/Foldable.hs
--- a/lib/Incipit/Foldable.hs
+++ b/lib/Incipit/Foldable.hs
@@ -6,7 +6,7 @@
 
 import qualified Data.Foldable as Foldable
 import Data.Foldable (
-  Foldable (elem, fold, foldMap, foldMap', foldl', foldl1, foldr, foldr', foldr1, length, null, product, sum, toList),
+  Foldable (elem, fold, foldMap, foldMap', foldl, foldl', foldl1, foldr, foldr', foldr1, length, null, product, sum, toList),
   all,
   and,
   any,
@@ -25,18 +25,6 @@
   )
 import Data.Maybe (Maybe (Just, Nothing))
 import Data.Ord (Ord, Ordering)
-
--- |Default to using the strict version since the lazy one is so controversial.
-foldl ::
-  ∀ t b a .
-  Foldable t =>
-  (b -> a -> b) ->
-  b ->
-  t a ->
-  b
-foldl =
-  Foldable.foldl'
-{-# inlineable foldl #-}
 
 -- |Safe version of 'Foldable.minimum'.
 minimum ::
diff --git a/lib/Incipit/List.hs b/lib/Incipit/List.hs
--- a/lib/Incipit/List.hs
+++ b/lib/Incipit/List.hs
@@ -29,5 +29,5 @@
 tail :: [a] -> Maybe [a]
 tail = \case
   [] -> Nothing
-  as -> Just (List.tail as)
+  _ : as -> Just as
 {-# inline tail #-}
diff --git a/lib/Incipit/String/Reexport.hs b/lib/Incipit/String/Reexport.hs
--- a/lib/Incipit/String/Reexport.hs
+++ b/lib/Incipit/String/Reexport.hs
@@ -1,4 +1,5 @@
 {-# options_haddock prune #-}
+{-# language CPP #-}
 
 {- |
 Copyright:  (c) 2016 Stephen Diehl
@@ -47,6 +48,9 @@
 import Text.Read (Read, readMaybe, reads)
 
 import Incipit.Base (Constraint, Type)
+#if __GLASGOW_HASKELL__ >= 904
+import Incipit.Base (type (~))
+#endif
 
 
 -- $setup
diff --git a/lib/Incipit/TypeError.hs b/lib/Incipit/TypeError.hs
--- a/lib/Incipit/TypeError.hs
+++ b/lib/Incipit/TypeError.hs
@@ -1,24 +1,39 @@
-{-# language StandaloneKindSignatures #-}
+{-# language StandaloneKindSignatures, CPP #-}
+#if __GLASGOW_HASKELL__ >= 910
+{-# language TypeAbstractions #-}
+#endif
 
 -- | Description: 'ErrorMessage' concatenation operators, stolen from type-errors-pretty.
 module Incipit.TypeError where
 
-import GHC.TypeLits (ErrorMessage (..), Symbol)
+import GHC.TypeLits (ErrorMessage (..), Symbol, TypeError)
 
+-- | Concat two error message strings.
 type (<>) :: k1 -> k2 -> ErrorMessage
 type family (<>) l r :: ErrorMessage where
   l <> r = ToErrorMessage l ':<>: ToErrorMessage r
 
 infixl 5 <>
 
+-- | Concat two error message strings with a newline.
 type (%) :: k1 -> k2 -> ErrorMessage
 type family (%) t b :: ErrorMessage where
   t % b = ToErrorMessage t ':$$: ToErrorMessage b
 
 infixr 4 %
 
+-- | Convert a 'Symbol', 'ErrorMessage' or type to an 'ErrorMessage'.
 type ToErrorMessage :: k -> ErrorMessage
 type family ToErrorMessage t where
   ToErrorMessage (t :: Symbol) = 'Text t
   ToErrorMessage (t :: ErrorMessage) = t
   ToErrorMessage t = 'ShowType t
+
+-- | Placeholder similar to 'IncipitBase.undefined'.
+type Undefined :: k
+#if __GLASGOW_HASKELL__ >= 910
+type family Undefined @k where
+#else
+type family Undefined where
+#endif
+  Undefined = TypeError ('Text "undefined")