diff --git a/CHANGELOG.markdown b/CHANGELOG.markdown
--- a/CHANGELOG.markdown
+++ b/CHANGELOG.markdown
@@ -1,3 +1,7 @@
+4.0.2
+-----
+* Added `assoc` to `Data.Profunctor.Composition` so that we have all 3 associators.
+
 4.0
 ---
 * Merged the contents of `profunctor-extras` into `profunctors`.
diff --git a/profunctors.cabal b/profunctors.cabal
--- a/profunctors.cabal
+++ b/profunctors.cabal
@@ -1,6 +1,6 @@
 name:          profunctors
 category:      Control, Categories
-version:       4.0.1
+version:       4.0.2
 license:       BSD3
 cabal-version: >= 1.10
 license-file:  LICENSE
diff --git a/src/Data/Profunctor.hs b/src/Data/Profunctor.hs
--- a/src/Data/Profunctor.hs
+++ b/src/Data/Profunctor.hs
@@ -179,6 +179,8 @@
 -- Minimal complete definition: 'first'' or 'second''
 --
 -- /Note:/ Every 'Functor' in Haskell is strong.
+--
+-- <http://takeichi.ipl-lab.org/~asada/papers/arrStrMnd.pdf>
 class Profunctor p => Strong p where
   first' :: p a b  -> p (a, c) (b, c)
   first' = dimap swap swap . second'
@@ -207,6 +209,7 @@
   second' (UpStar f) = UpStar $ \ ~(c, a) -> (,) c <$> f a
   {-# INLINE second' #-}
 
+-- | Every Arrow is a Strong Monad in Prof
 instance Arrow p => Strong (WrappedArrow p) where
   first' (WrapArrow k) = WrapArrow (first k)
   {-# INLINE first' #-}
diff --git a/src/Data/Profunctor/Composition.hs b/src/Data/Profunctor/Composition.hs
--- a/src/Data/Profunctor/Composition.hs
+++ b/src/Data/Profunctor/Composition.hs
@@ -21,9 +21,10 @@
   -- * Profunctor Composition
     Procompose(..)
   , procomposed
-  -- * Lax identity
+  -- * Bicategorical Associators
   , idl
   , idr
+  , assoc
   -- * Generalized Composition
   , upstars, kleislis
   , downstars, cokleislis
@@ -128,6 +129,18 @@
 -- @
 idr :: Profunctor q => Iso (Procompose q (->) d c) (Procompose r (->) d' c') (q d c) (r d' c')
 idr = dimap (\(Procompose f g) -> rmap g f) (fmap (`Procompose` id))
+
+
+-- | The associator for 'Profunctor' composition.
+--
+-- This provides an 'Iso' for the @lens@ package that witnesses the
+-- isomorphism between @'Procompose' p ('Procompose' q r) a b@ and
+-- @'Procompose' ('Procompose' p q) r a b@, which arises because
+-- @Prof@ is only a bicategory, rather than a strict 2-category.
+assoc :: Iso (Procompose p (Procompose q r) a b) (Procompose x (Procompose y z) a b)
+             (Procompose (Procompose p q) r a b) (Procompose (Procompose x y) z a b)
+assoc = dimap (\(Procompose f (Procompose g h)) -> Procompose (Procompose f g) h)
+              (fmap (\(Procompose (Procompose f g) h) -> Procompose f (Procompose g h)))
 
 -- | 'Profunctor' composition generalizes 'Functor' composition in two ways.
 --