diff --git a/Presentation/Yeamer.hs b/Presentation/Yeamer.hs
--- a/Presentation/Yeamer.hs
+++ b/Presentation/Yeamer.hs
@@ -24,6 +24,7 @@
 {-# LANGUAGE TupleSections          #-}
 {-# LANGUAGE ConstraintKinds        #-}
 {-# LANGUAGE ViewPatterns           #-}
+{-# LANGUAGE Rank2Types             #-}
 
 module Presentation.Yeamer ( Presentation
                            -- * Running a presentation
@@ -40,8 +41,10 @@
                            , verbatim, plaintext, verbatimWithin
                            -- * Structure / composition
                            , addHeading, (======), discardResult
+                           , module Data.Monoid
+                           , module Data.Semigroup.Numbered
                            -- * CSS
-                           , divClass, spanClass, (#%), styling
+                           , divClass, divClasses, spanClass, (#%), styling, Css
                            -- * Server configuration
                            , yeamer'
                            , YeamerServerConfig
@@ -140,11 +143,19 @@
 instance (Flat a) => Flat (Identity a)
 #endif
 
+type Sessionable = Flat
+
+data SessionableWitness a where
+  SessionableWitness :: Sessionable a => SessionableWitness a
+data EncapsulableWitness t where
+  EncapsulableWitness :: (∀ r . Sessionable r => SessionableWitness (t r))
+                           -> EncapsulableWitness t
+
 data Container t where
   WithHeading :: Html -> Container Identity
   ManualCSSClasses :: Container (WriterT HTMChunkK [])
   GriddedBlocks :: Container Gridded
-  CustomEncapsulation :: (t Html -> Html) -> Container t
+  CustomEncapsulation :: EncapsulableWitness t -> (t Html -> Html) -> Container t
 
 data HTMChunkK = HTMDiv {_hchunkCSSClass::Text} | HTMSpan {_hchunkCSSClass::Text}
           deriving (Generic, Eq, Ord)
@@ -153,8 +164,6 @@
 instance Flat HTMChunkK
 makeLenses ''HTMChunkK
 
-type Sessionable = Flat
-
 data IPresentation m r where
    StaticContent :: Html -> IPresentation m ()
    Resultless :: IPresentation m r -> IPresentation m ()
@@ -202,8 +211,8 @@
 preprocPres (Styling s p) = Styling s $ preprocPres p
 preprocPres (Encaps (WithHeading h) p) = Encaps (WithHeading h) $ preprocPres<$>p
 preprocPres (Encaps ManualCSSClasses p) = Encaps ManualCSSClasses $ preprocPres<$>p
-preprocPres (Encaps (CustomEncapsulation f) p)
-                = Encaps (CustomEncapsulation f) $ preprocPres<$>p
+preprocPres (Encaps (CustomEncapsulation (EncapsulableWitness w) f) p)
+                = Encaps (CustomEncapsulation (EncapsulableWitness w) f) $ preprocPres<$>p
 preprocPres (Encaps GriddedBlocks p)
            = Styling grids
            . divClass gridClass
@@ -388,12 +397,13 @@
        go lvl (Encaps (WithHeading h) conts)
            = let lvl' = min 6 $ lvl + 1
                  hh = [HTM.h1, HTM.h2, HTM.h3, HTM.h4, HTM.h5, HTM.h6]!!lvl
-             in go lvl' $ Encaps (CustomEncapsulation $ \(Identity contsr)
+             in go lvl' $ Encaps (CustomEncapsulation (EncapsulableWitness SessionableWitness)
+                                         $ \(Identity contsr)
                                     -> HTM.div HTM.! HTM.class_ "headed-container"
                                          $ hh h <> contsr
                                  ) conts
        go lvl (Encaps ManualCSSClasses conts)
-           = go lvl $ Encaps (CustomEncapsulation $ \(WriterT contsrs)
+           = go lvl $ Encaps (CustomEncapsulation (EncapsulableWitness SessionableWitness) $ \(WriterT contsrs)
                   -> foldMap (\(q,i) -> case i of
                                HTMDiv c -> [hamlet| <div class=#{withSupclass c}> #{q} |]()
                                HTMSpan c -> [hamlet| <span class=#{withSupclass c}> #{q} |]())
@@ -403,7 +413,7 @@
                | Just _ <- Txt.stripPrefix "autogrid_" c
                             = "autogrid "<>c
                | otherwise  = c
-       go lvl (Encaps (CustomEncapsulation f) conts) = f $ go lvl <$> conts
+       go lvl (Encaps (CustomEncapsulation (EncapsulableWitness _) f) conts) = f $ go lvl <$> conts
        go _ p = error $ outerConstructorName p <> " cannot be rendered."
 
 
@@ -468,7 +478,7 @@
 outerConstructorName (Resultless _) = "Resultless"
 outerConstructorName (Styling _ _) = "Styling"
 outerConstructorName (Encaps (WithHeading _) _) = "Encaps WithHeading"
-outerConstructorName (Encaps (CustomEncapsulation _) _) = "Encaps CustomEncapsulation"
+outerConstructorName (Encaps (CustomEncapsulation _ _) _) = "Encaps CustomEncapsulation"
 outerConstructorName (Encaps ManualCSSClasses _) = "Encaps ManualCSSClasses"
 outerConstructorName (Encaps GriddedBlocks _) = "Encaps GriddedBlocks"
 outerConstructorName (Pure _) = "Pure"
@@ -494,7 +504,7 @@
   f <*> Pure x = fmap ($ x) f
   fs<*>xs = ap fs xs
 
-instance Monad (IPresentation m) where
+instance ∀ m . Monad (IPresentation m) where
   return = pure
   StaticContent c >>= f = Dependent (StaticContent c) f
   Resultless p >>= f = Dependent (Resultless p) f
@@ -511,6 +521,19 @@
   Styling s (Dependent p g) >>= f = Dependent (Styling s p) $ Styling s . g >=> f
   Encaps (WithHeading h) p >>= f = Dependent (Encaps (WithHeading h) p) f
   Encaps ManualCSSClasses ps >>= f = Dependent (Encaps ManualCSSClasses ps) f
+  Encaps (CustomEncapsulation (EncapsulableWitness w') e') ps' >>= f'
+      = bindCustEncaps w' e' ps' f'
+   where bindCustEncaps :: ∀ a b t r
+                        . (Sessionable r, Traversable t, Sessionable (t ()))
+                        => (∀ r' . Sessionable r' => SessionableWitness (t r'))
+                        -> (t Html -> Html)
+                        -> t (IPresentation m r)
+                        -> (t r -> IPresentation m b)
+                        -> IPresentation m b
+         bindCustEncaps w e ps f
+          = case w :: SessionableWitness (t r) of
+             SessionableWitness
+               -> Dependent (Encaps (CustomEncapsulation (EncapsulableWitness w) e) ps) f
   Pure x >>= f = f x
   Deterministic g p >>= f = p >>= f . g
   Interactive p q >>= f = Dependent (Interactive p q) f
@@ -537,6 +560,11 @@
 spanClass cn = fmap (fst . head . runWriterT)
               . Encaps ManualCSSClasses . WriterT . pure . (,HTMSpan cn)
 
+divClasses :: Sessionable r => [(Text, IPresentation m r)] -> IPresentation m r
+divClasses cns = fmap (fst . head . runWriterT)
+              . Encaps ManualCSSClasses $ WriterT [ (content,HTMDiv cn)
+                                                  | (cn,content) <- cns ]
+
 infix 8 #%
 -- | Assign this content a CSS class attribute. If the content is inline, this will
 --   be a @<span>@, else a @<div>@.
@@ -554,7 +582,8 @@
 
 tweakContent :: Sessionable r => (Html -> Html) -> IPresentation m r -> IPresentation m r
 tweakContent f = fmap runIdentity
-               . Encaps (CustomEncapsulation $ f . runIdentity)
+               . Encaps (CustomEncapsulation (EncapsulableWitness SessionableWitness)
+                              $ f . runIdentity)
                . Identity
 
 infixr 6 $<>
@@ -730,7 +759,7 @@
              . Just <$> liftIO q
        go crumbs path (Encaps (WithHeading _) (Identity cont))
            = (,True) . fmap Identity . fst <$> go crumbs path cont
-       go crumbs path (Encaps (CustomEncapsulation _) cont)
+       go crumbs path (Encaps (CustomEncapsulation _ _) cont)
            = (,True) . sequence
               <$> traverse (\c -> fst <$> go crumbs path c) cont
        go (crumbh,choiceName,crumbp) [] (Encaps ManualCSSClasses (WriterT conts))
diff --git a/demo/Main.hs b/demo/Main.hs
--- a/demo/Main.hs
+++ b/demo/Main.hs
@@ -6,9 +6,6 @@
 
 import Text.Cassius
 
-import Data.Semigroup
-import Data.Semigroup.Numbered
-
 import qualified Diagrams.Prelude as Dia
 import qualified Diagrams.Backend.Cairo as Dia
 
diff --git a/test/Main.hs b/test/Main.hs
--- a/test/Main.hs
+++ b/test/Main.hs
@@ -8,8 +8,6 @@
 import Text.Hamlet
 
 import Data.Foldable
-import Data.Semigroup
-import Data.Semigroup.Numbered
 import Data.String (fromString)
 import Data.Function (fix)
 
diff --git a/yeamer.cabal b/yeamer.cabal
--- a/yeamer.cabal
+++ b/yeamer.cabal
@@ -2,7 +2,7 @@
 -- documentation, see http://haskell.org/cabal/users-guide/
 
 name:                yeamer
-version:             0.1.0.3
+version:             0.1.0.4
 synopsis:            Yesod-based server for interactive presentation slides
 -- description:         
 homepage:            https://github.com/leftaroundabout/yeamer