diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,18 @@
+# Changelog
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to the [Haskell Package Versioning Policy](https://pvp.haskell.org/).
+
+## 0.2.0.0 – 2019–01–08
+### Added
+- `DFunctor` instances for `Mu` and `Nu`
+- lower bounds on internal yaya dependencies
+
+### Changed
+- moved `DFunctor` and `HFunctor` to a new `Yaya.Functor` module
+- renamed `cursiveIso` to `steppableIso`
+
+## 0.1.0.0 – 2019–01–04
+### Added
+- everything (this is the initial release)
diff --git a/src/Yaya/Fold.hs b/src/Yaya/Fold.hs
--- a/src/Yaya/Fold.hs
+++ b/src/Yaya/Fold.hs
@@ -24,6 +24,7 @@
 import Numeric.Natural
 
 import Yaya.Fold.Common
+import Yaya.Functor
 import Yaya.Pattern
 
 type Algebra f a = f a -> a
@@ -79,7 +80,6 @@
 -- | A fixed-point operator for inductive / finite data structures.
 data Mu f = Mu (forall a. Algebra f a -> a)
 
-
 instance Functor f => Projectable (Mu f) f where
   project = lambek
 
@@ -89,6 +89,9 @@
 instance Recursive (Mu f) f where
   cata φ (Mu f) = f φ
 
+instance DFunctor Mu where
+ dmap f (Mu fold) = Mu (\φ -> fold (φ . f))
+
 instance Show1 f => Show (Mu f) where
   showsPrec = recursiveShowsPrec
 
@@ -108,6 +111,9 @@
 instance Corecursive (Nu f) f where
   ana = Nu
 
+instance DFunctor Nu where
+  dmap f (Nu φ a) = Nu (f . φ) a
+
 instance Projectable [a] (XNor a) where
   project []      = Neither
   project (h : t) = Both h t
@@ -309,6 +315,11 @@
 ignoringAttribute :: Algebra f a -> Algebra (EnvT b f) a
 ignoringAttribute φ = φ . lowerEnvT
 
+-- | It is somewhat common to have a natural transformation that looks like
+--  `η :: forall a. f a -> Free g a`. This maps naturally to a `GCoalgebra` (to
+--   pass to `apo`) with `η . project`, but the desired `Algebra` is more likely
+--   to be `cata unFree . η` than `embed . η`. See yaya-streams for some
+--   examples of this.
 unFree :: Steppable t f => Algebra (FreeF f t) t
 unFree = \case
   Pure t  -> t
@@ -317,7 +328,7 @@
 -- preservingAttribute :: (forall a. f a -> g a) -> EnvT a f b -> EnvT a g b
 -- preservingAttribute = cohoist
 
--- instances for non-recursive types
+-- * instances for non-recursive types
 
 constEmbed :: Algebra (Const a) a
 constEmbed = getConst
@@ -355,30 +366,14 @@
 instance Corecursive (Maybe a) (Const (Maybe a)) where
   ana = constAna
 
--- | An endofunctor in the category of endofunctors.
-class HFunctor h where
-  hmap :: (forall a. f a -> g a) -> h f a -> h g a
-
-
--- | A functor from the category of endofunctors to *Hask*.
-class DFunctor d where
-  dmap :: (forall a. f a -> g a) -> d f -> d g
-
--- instance DFunctor Mu where
---   dmap f = cata (embed . f)
-
--- instance DFunctor Fix where
---   dmap f = ana (f . project)
-
--- instance DFunctor Nu where
---   dmap f = ana (f . project)
+-- * Optics
 
 type BialgebraIso f a = Iso' (f a) a
 type AlgebraPrism f a = Prism' (f a) a
 type CoalgebraPrism f a = Prism' a (f a)
 
-cursiveIso :: Steppable t f => BialgebraIso f t
-cursiveIso = iso embed project
+steppableIso :: Steppable t f => BialgebraIso f t
+steppableIso = iso embed project
 
 birecursiveIso
   :: (Recursive t f, Corecursive t f)
diff --git a/src/Yaya/Fold/Common.hs b/src/Yaya/Fold/Common.hs
--- a/src/Yaya/Fold/Common.hs
+++ b/src/Yaya/Fold/Common.hs
@@ -36,6 +36,19 @@
   liftEq (==) (void f1) (void f2)
   && and (zipWith fn (toList f1) (toList f2))
 
+-- -- | What we want here is a way we can get a
+-- --  `corecursiveEq :: t -> t -> Partial Bool`, so we can define (unsafe) `Eq`
+-- --   instances with `eq = runToEnd corecursiveEq`.
+-- coequal :: (Functor f, Foldable f, Eq1 f) => t -> t -> Either Bool [(t, t)]
+-- coequal a b =
+--   let fa = project a
+--       fb = project b
+--   in if liftEq (==) (void fa) (void fb)
+--      then case zip (toList fa) (toList fb) of
+--             [] -> Left True
+--             ps -> Right ps
+--      else Left False
+
 -- TODO: Redefine this using `Natural`
 -- | When folded, returns the height of the data structure.
 height :: Foldable f => f Integer -> Integer
diff --git a/src/Yaya/Functor.hs b/src/Yaya/Functor.hs
new file mode 100644
--- /dev/null
+++ b/src/Yaya/Functor.hs
@@ -0,0 +1,22 @@
+-- | This should probably be a separate library, but it provides a number of
+--   functor type classes between various categories.
+module Yaya.Functor where
+
+import Data.Bifunctor
+
+-- | A functor from the category of endofunctors to *Hask*. The `D` is meant to
+--   be a mnemonic for “down”, as we’re “lowering” from endofunctors to types.
+class DFunctor (d :: (* -> *) -> *) where
+  dmap :: (forall a. f a -> g a) -> d f -> d g
+
+-- | This isn’t a Functor instance because of the position of the `a`, but you
+--   can use it like:
+--   > newtype List a = List (Mu (XNor a))
+--   > instance Functor List where
+--   >   map f (List mu) = List (firstMap f mu)
+firstMap :: (DFunctor d, Bifunctor f) => (a -> b) -> d (f a) -> d (f b)
+firstMap f = dmap (first f)
+
+-- | An endofunctor in the category of endofunctors.
+class HFunctor (h :: (* -> *) -> * -> *) where
+  hmap :: (forall a. f a -> g a) -> h f a -> h g a
diff --git a/test/Test/Fold.hs b/test/Test/Fold.hs
--- a/test/Test/Fold.hs
+++ b/test/Test/Fold.hs
@@ -21,5 +21,9 @@
 -- prop_muCataCompose =
 --   property $ law_cataCompose size id =<< forAll genMuExpr
 
+-- prop_nuAnaCancel :: Property
+-- prop_nuAnaCancel =
+--   property $ law_anaCancel size =<< forAll (genExpr (Gen.sized genNuExpr))
+
 tests :: IO Bool
 tests = checkParallel $$(discover)
diff --git a/yaya.cabal b/yaya.cabal
--- a/yaya.cabal
+++ b/yaya.cabal
@@ -1,5 +1,5 @@
 name:                yaya
-version:             0.1.0.0
+version:             0.2.0.0
 synopsis:            Total recursion schemes.
 description:         Recursion schemes allow you to separate recursion from your
                      business logic – making your own operations simpler, more
@@ -15,7 +15,8 @@
 license-file:        LICENSE
 category:            Recursion
 build-type:          Simple
-extra-source-files:  README.md
+extra-source-files:  CHANGELOG.md
+                   , README.md
 cabal-version:       >=1.10
 
 library
@@ -24,6 +25,7 @@
                      , Yaya.Fold
                      , Yaya.Fold.Common
                      , Yaya.Fold.Native
+                     , Yaya.Functor
                      , Yaya.Retrofit
                      , Yaya.Applied
                      , Yaya.Zoo
@@ -64,8 +66,8 @@
   build-depends:       base
                      , deriving-compat
                      , hedgehog
-                     , yaya
-                     , yaya-hedgehog
+                     , yaya >= 0.1.0
+                     , yaya-hedgehog >= 0.1.0
   ghc-options:         -threaded -rtsopts -with-rtsopts=-N -Wall
   default-language:    Haskell2010