diff --git a/colonnade.cabal b/colonnade.cabal
--- a/colonnade.cabal
+++ b/colonnade.cabal
@@ -1,5 +1,5 @@
 name:                colonnade
-version:             1.1.0
+version:             1.1.1
 synopsis:            Generic types and functions for columnar encoding and decoding
 description:
   The `colonnade` package provides a way to to talk about
@@ -39,6 +39,7 @@
     , text >= 1.0 && < 1.3
     , bytestring >= 0.10 && < 0.11
     , profunctors >= 4.0 && < 5.3
+    , semigroups >= 0.17 && < 0.19
   default-language:    Haskell2010
   ghc-options: -Wall
 
diff --git a/src/Colonnade.hs b/src/Colonnade.hs
--- a/src/Colonnade.hs
+++ b/src/Colonnade.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE DataKinds #-}
+{-# LANGUAGE RankNTypes #-}
 
-{-# OPTIONS_GHC -Wall -fno-warn-unused-imports -fno-warn-unticked-promoted-constructors -Werror #-}
+{-# OPTIONS_GHC -Wall -fno-warn-unused-imports -fno-warn-unticked-promoted-constructors #-}
 
 -- | Build backend-agnostic columnar encodings that can be 
 --   used to visualize tabular data.
@@ -16,12 +17,16 @@
   , headless
   , singleton
     -- * Transform
-  , mapHeaderContent
+    -- ** Body
   , fromMaybe
   , columns
   , bool
   , replaceWhen
   , modifyWhen
+    -- ** Header
+  , mapHeaderContent
+  , mapHeadedness
+  , toHeadless
     -- * Cornice
     -- ** Types
   , Cornice
@@ -118,6 +123,16 @@
 mapHeaderContent f (E.Colonnade v) = 
   E.Colonnade (Vector.map (\(E.OneColonnade h e) -> E.OneColonnade (fmap f h) e) v)
 
+-- | Map over the header type of a 'Colonnade'.
+mapHeadedness :: (forall x. h x -> h' x) -> Colonnade h a c -> Colonnade h' a c
+mapHeadedness f (E.Colonnade v) =
+  E.Colonnade (Vector.map (\(E.OneColonnade h e) -> E.OneColonnade (f h) e) v)
+
+-- | Remove the heading from a 'Colonnade'.
+toHeadless :: Colonnade h a c -> Colonnade Headless a c
+toHeadless = mapHeadedness (const Headless)
+  
+
 -- | Lift a column over a 'Maybe'. For example, if some people
 --   have houses and some do not, the data that pairs them together
 --   could be represented as:
@@ -216,12 +231,7 @@
   -> (a -> Bool) -- ^ Row predicate
   -> Colonnade f a c -- ^ Original 'Colonnade'
   -> Colonnade f a c
-replaceWhen newContent p (E.Colonnade v) = E.Colonnade
-  ( Vector.map
-    (\(E.OneColonnade h encode) -> E.OneColonnade h $ \a ->
-      if p a then newContent else encode a
-    ) v
-  )
+replaceWhen = modifyWhen . const
 
 -- | Augment a 'Colonnade' with a header spans over all of the
 --   existing headers. This is best demonstrated by example. 
diff --git a/src/Colonnade/Encode.hs b/src/Colonnade/Encode.hs
--- a/src/Colonnade/Encode.hs
+++ b/src/Colonnade/Encode.hs
@@ -8,7 +8,7 @@
 {-# LANGUAGE ScopedTypeVariables #-}
 
 {-# OPTIONS_HADDOCK not-home #-}
-{-# OPTIONS_GHC -Wall -fno-warn-unused-imports -fno-warn-unticked-promoted-constructors -Werror #-}
+{-# OPTIONS_GHC -Wall -fno-warn-unused-imports -fno-warn-unticked-promoted-constructors #-}
 
 -- | Most users of this library do not need this module. The functions
 --   here are used to build functions that apply a 'Colonnade'
diff --git a/test/Main.hs b/test/Main.hs
--- a/test/Main.hs
+++ b/test/Main.hs
@@ -2,5 +2,5 @@
 
 main :: IO ()
 main = doctest
-  [ "src/Colonnade.hs"
+  [ "src"
   ]