diff --git a/semantic-diff.cabal b/semantic-diff.cabal
index c8d8899de..63d92519d 100644
--- a/semantic-diff.cabal
+++ b/semantic-diff.cabal
@@ -18,6 +18,7 @@ library
                      , Category
                      , Data.Bifunctor.Join.Arbitrary
                      , Data.Functor.Both
+                     , Data.RandomWalkSimilarity
                      , Data.OrderedMap
                      , Data.Record
                      , Data.These.Arbitrary
@@ -55,8 +56,12 @@ library
                      , bytestring
                      , containers
                      , directory
+                     , dlist
                      , filepath
+                     , hashable
+                     , kdt
                      , mtl
+                     , MonadRandom
                      , pointed
                      , QuickCheck >= 2.8.1
                      , quickcheck-text
@@ -71,7 +76,7 @@ library
                      , comonad
                      , protolude
   default-language:    Haskell2010
-  default-extensions:  DeriveFunctor, DeriveFoldable, DeriveTraversable, DeriveGeneric, FlexibleInstances, OverloadedStrings, NoImplicitPrelude, RecordWildCards
+  default-extensions:  DeriveFunctor, DeriveFoldable, DeriveTraversable, DeriveGeneric, FlexibleContexts, FlexibleInstances, OverloadedStrings, NoImplicitPrelude, RecordWildCards
   ghc-options:        -Wall -fno-warn-name-shadowing -O2 -threaded -fprof-auto "-with-rtsopts=-N -p -s -h -i0.1" -j
 
 benchmark semantic-diff-bench
@@ -95,18 +100,20 @@ test-suite semantic-diff-test
   hs-source-dirs:      test
   main-is:             Spec.hs
   other-modules:       AlignmentSpec
-                     , ArbitraryTerm
                      , CorpusSpec
+                     , Data.RandomWalkSimilarity.Spec
+                     , Diff.Spec
+                     , DiffSummarySpec
                      , InterpreterSpec
                      , OrderedMapSpec
                      , PatchOutputSpec
                      , TermSpec
-                     , DiffSummarySpec
   build-depends:       base
                      , bifunctors
                      , bytestring
                      , containers
                      , deepseq
+                     , dlist
                      , filepath
                      , Glob
                      , hspec >= 2.1.10
diff --git a/src/Alignment.hs b/src/Alignment.hs
index c6bd1561a..2e16e99dc 100644
--- a/src/Alignment.hs
+++ b/src/Alignment.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE FlexibleContexts, RankNTypes, ScopedTypeVariables #-}
+{-# LANGUAGE RankNTypes, ScopedTypeVariables #-}
 module Alignment
 ( hasChanges
 , numberedRows
diff --git a/src/Category.hs b/src/Category.hs
index f621177dc..5dd35d817 100644
--- a/src/Category.hs
+++ b/src/Category.hs
@@ -1,7 +1,9 @@
 module Category where
 
 import Prologue
+import Data.Hashable
 import Data.String
+import Test.QuickCheck
 
 -- | A standardized category of AST node. Used to determine the semantics for
 -- | semantic diffing and define comparability of nodes.
@@ -28,4 +30,27 @@ data Category
   | ArrayLiteral
   -- | A non-standard category, which can be used for comparability.
   | Other String
-  deriving (Eq, Show, Ord)
+  deriving (Eq, Generic, Ord, Show)
+
+
+-- Instances
+
+instance Hashable Category
+
+instance Arbitrary Category where
+  arbitrary = oneof
+    [ pure Program
+    , pure Error
+    , pure BinaryOperator
+    , pure DictionaryLiteral
+    , pure Pair
+    , pure FunctionCall
+    , pure StringLiteral
+    , pure IntegerLiteral
+    , pure SymbolLiteral
+    , pure ArrayLiteral
+    , Other <$> arbitrary
+    ]
+
+  shrink (Other s) = Other <$> shrink s
+  shrink _ = []
diff --git a/src/Data/Functor/Both.hs b/src/Data/Functor/Both.hs
index 3e4cbfae8..544b7223a 100644
--- a/src/Data/Functor/Both.hs
+++ b/src/Data/Functor/Both.hs
@@ -1,3 +1,4 @@
+{-# OPTIONS_GHC -fno-warn-orphans #-}
 module Data.Functor.Both (Both,both, runBothWith, fst, snd, module X) where
 
 import Data.Bifunctor.Join as X
diff --git a/src/Data/OrderedMap.hs b/src/Data/OrderedMap.hs
index 5c4eb5b26..fe789a38a 100644
--- a/src/Data/OrderedMap.hs
+++ b/src/Data/OrderedMap.hs
@@ -15,10 +15,11 @@ module Data.OrderedMap (
   ) where
 
 import Prologue hiding (toList, empty)
+import Test.QuickCheck
 
 -- | An ordered map of keys and values.
 newtype OrderedMap key value = OrderedMap { toList :: [(key, value)] }
-  deriving (Show, Eq, Functor, Foldable, Traversable)
+  deriving (Eq, Foldable, Functor, Generic, Ord, Show, Traversable)
 
 -- | Construct an ordered map from a list of pairs of keys and values.
 fromList :: [(key, value)] -> OrderedMap key value
@@ -70,3 +71,7 @@ difference (OrderedMap a) (OrderedMap b) = OrderedMap $ filter ((`notElem` extan
 instance Eq key => Monoid (OrderedMap key value) where
   mempty = fromList []
   mappend = union
+
+instance (Arbitrary key, Arbitrary value) => Arbitrary (OrderedMap key value) where
+  arbitrary = fromList <$> arbitrary
+  shrink = genericShrink
diff --git a/src/Data/RandomWalkSimilarity.hs b/src/Data/RandomWalkSimilarity.hs
new file mode 100644
index 000000000..bf44d7939
--- /dev/null
+++ b/src/Data/RandomWalkSimilarity.hs
@@ -0,0 +1,112 @@
+module Data.RandomWalkSimilarity where
+
+import Control.Arrow ((&&&))
+import Control.Monad.Random
+import Control.Monad.State
+import Data.Bifunctor.Join
+import qualified Data.DList as DList
+import Data.Functor.Foldable as Foldable
+import Data.Hashable
+import qualified Data.KdTree.Static as KdTree
+import qualified Data.List as List
+import qualified Data.OrderedMap as Map
+import qualified Data.Vector as Vector
+import Diff
+import Patch
+import Prologue
+import Syntax
+import Term
+import Test.QuickCheck hiding (Fixed)
+import Test.QuickCheck.Random
+
+-- | Given a function comparing two terms recursively, and a function to compute a Hashable label from an annotation, compute the diff of a pair of lists of terms using a random walk similarity metric, which completes in log-linear time. This implementation is based on the paper [_RWS-Diff—Flexible and Efficient Change Detection in Hierarchical Data_](https://github.com/github/semantic-diff/files/325837/RWS-Diff.Flexible.and.Efficient.Change.Detection.in.Hierarchical.Data.pdf).
+rws :: (Hashable label, Hashable leaf, Eq leaf, Ord annotation) => (Term leaf annotation -> Term leaf annotation -> Maybe (Diff leaf annotation)) -> (annotation -> label) -> [Term leaf annotation] -> [Term leaf annotation] -> [Diff leaf annotation]
+rws compare getLabel as bs
+  | null as, null bs = []
+  | null as = insert <$> bs
+  | null bs = delete <$> as
+  | otherwise = uncurry deleteRemaining . (`runState` fas) $ traverse findNearestNeighbourTo fbs
+  where insert = pure . Insert
+        delete = pure . Delete
+        replace = (pure .) . Replace
+        (p, q, d) = (2, 2, 15)
+        fas = featurize <$> as
+        fbs = featurize <$> bs
+        kdas = KdTree.build (Vector.toList . fst) fas
+        featurize = featureVector d . pqGrams p q getLabel &&& identity
+        findNearestNeighbourTo kv@(_, v) = do
+          unmapped <- get
+          let (k, _) = KdTree.nearest kdas kv
+          case k `List.lookup` unmapped of
+            Nothing -> pure $! insert v
+            Just found -> do
+              put (List.delete (k, found) unmapped)
+              pure $! fromMaybe (replace found v) (compare found v)
+        deleteRemaining diffs unmapped = foldl' (flip (List.insertBy (comparing firstAnnotation))) diffs (delete . snd <$> unmapped)
+
+-- | Extract the annotation for the before state of a diff node. This is returned in `Maybe` because e.g. an `Insert` patch does not have an annotation for the before state.
+firstAnnotation :: Diff leaf annotation -> Maybe annotation
+firstAnnotation diff = case runFree diff of
+  Free (annotations :< _) -> Just (fst (runJoin annotations))
+  Pure patch -> maybeFst (unPatch $ extract <$> patch)
+
+-- | A `Gram` is a fixed-size view of some portion of a tree, consisting of a `stem` of _p_ labels for parent nodes, and a `base` of _q_ labels of sibling nodes. Collectively, the bag of `Gram`s for each node of a tree (e.g. as computed by `pqGrams`) form a summary of the tree.
+data Gram label = Gram { stem :: [Maybe label], base :: [Maybe label] }
+  deriving (Eq, Show)
+
+-- | Compute the bag of grams with stems of length _p_ and bases of length _q_, with labels computed from annotations, which summarize the entire subtree of a term.
+pqGrams :: Int -> Int -> (annotation -> label) -> Cofree (Syntax leaf) annotation -> DList.DList (Gram (label, Maybe leaf))
+pqGrams p q getLabel = cata merge . setRootBase . setRootStem . hylo go project
+  where go (annotation :< functor) = cofree (Gram [] [ Just (getLabel annotation, leafValue functor) ] :< (assignParent (Just (getLabel annotation, leafValue functor)) p <$> functor))
+        leafValue (Leaf s) = Just s
+        leafValue _ = Nothing
+        merge (head :< tail) = DList.cons head (Prologue.fold tail)
+        assignParent parentLabel n tree
+          | n > 0 = let gram :< functor = runCofree tree in cofree $ prependParent parentLabel gram :< assignSiblings (assignParent parentLabel (pred n) <$> functor)
+          | otherwise = tree
+        prependParent parentLabel gram = gram { stem = parentLabel : stem gram }
+        assignSiblings functor = case functor of
+          Leaf a -> Leaf a
+          Indexed a -> Indexed $ windowed q setBases [] a
+          Fixed a -> Fixed $ windowed q setBases [] a
+          Keyed a -> Keyed . Map.fromList $ windowed q setBasesKV [] (Map.toList a)
+        setBases child siblings rest = let (gram :< further) = (runCofree child) in cofree (setBase gram (siblings >>= base . extract) :< further) : rest
+        setBasesKV (key, child) siblings rest = let (gram :< further) = (runCofree child) in (key, cofree (setBase gram (siblings >>= base . extract . snd) :< further)) : rest
+        setBase gram newBase = gram { base = take q (newBase <> repeat Nothing) }
+        setRootBase term = let (a :< f) = runCofree term in cofree (setBase a (base a) :< f)
+        setRootStem = foldr (\ p rest -> assignParent Nothing p . rest) identity [0..p]
+
+-- | A sliding-window fold over _n_ items of a list per iteration.
+windowed :: Int -> (a -> [a] -> b -> b) -> b -> [a] -> b
+windowed n f seed = para alg
+  where alg xs = case xs of
+          Cons a (as, b) -> f a (take n $ a : as) b
+          Nil -> seed
+
+
+-- | Compute a vector with the specified number of dimensions, as an approximation of a bag of `Gram`s summarizing a tree.
+featureVector :: Hashable label => Int -> DList.DList (Gram label) -> Vector.Vector Double
+featureVector d bag = sumVectors $ unitDVector . hash <$> bag
+  where unitDVector hash = normalize . (`evalRand` mkQCGen hash) $ Prologue.sequence (Vector.replicate d getRandom)
+        normalize vec = fmap (/ vmagnitude vec) vec
+        sumVectors = DList.foldr (Vector.zipWith (+)) (Vector.replicate d 0)
+
+-- | The magnitude of a Euclidean vector, i.e. its distance from the origin.
+vmagnitude :: Vector.Vector Double -> Double
+vmagnitude = sqrtDouble . Vector.sum . fmap (** 2)
+
+
+-- Instances
+
+instance Hashable label => Hashable (Gram label) where
+  hashWithSalt _ = hash
+  hash gram = hash (stem gram <> base gram)
+
+-- | Construct a generator for arbitrary `Gram`s of size `(p, q)`.
+gramWithPQ :: Arbitrary label => Int -> Int -> Gen (Gram label)
+gramWithPQ p q = Gram <$> vectorOf p arbitrary <*> vectorOf q arbitrary
+
+instance Arbitrary label => Arbitrary (Gram label) where
+  arbitrary = join $ gramWithPQ <$> arbitrary <*> arbitrary
+
+  shrink (Gram a b) = Gram <$> shrink a <*> shrink b
diff --git a/src/Data/Record.hs b/src/Data/Record.hs
index 03127f39a..5d6043771 100644
--- a/src/Data/Record.hs
+++ b/src/Data/Record.hs
@@ -1,7 +1,8 @@
-{-# LANGUAGE DataKinds, FlexibleContexts, GADTs, KindSignatures, MultiParamTypeClasses, TypeOperators #-}
+{-# LANGUAGE DataKinds, GADTs, KindSignatures, MultiParamTypeClasses, TypeOperators #-}
 module Data.Record where
 
 import Prologue
+import Test.QuickCheck
 
 -- | A type-safe, extensible record structure.
 -- |
@@ -50,3 +51,17 @@ instance (Eq h, Eq (Record t)) => Eq (Record (h ': t)) where
 
 instance Eq (Record '[]) where
   _ == _ = True
+
+
+instance (Ord h, Ord (Record t)) => Ord (Record (h ': t)) where
+  RCons h1 t1 `compare` RCons h2 t2 = let h = h1 `compare` h2 in
+    if h == EQ then t1 `compare` t2 else h
+
+instance Ord (Record '[]) where
+  _ `compare` _ = EQ
+
+
+instance Arbitrary fields => Arbitrary (Record '[fields]) where
+  arbitrary = RCons <$> arbitrary <*> pure RNil
+
+  shrink (RCons h t) = RCons <$> shrink h <*> pure t
diff --git a/src/Diff.hs b/src/Diff.hs
index d95cd16ab..87c727d99 100644
--- a/src/Diff.hs
+++ b/src/Diff.hs
@@ -1,9 +1,11 @@
 {-# LANGUAGE TypeFamilies, TypeSynonymInstances #-}
+{-# OPTIONS_GHC -fno-warn-orphans #-}
 module Diff where
 
 import Prologue
 import Data.Functor.Foldable as Foldable
-import Data.Functor.Both
+import Data.Functor.Both as Both
+import qualified Data.OrderedMap as Map
 import Patch
 import Syntax
 import Term
@@ -13,8 +15,8 @@ type DiffF leaf annotation = FreeF (CofreeF (Syntax leaf) (Both annotation)) (Pa
 type Diff a annotation = Free (CofreeF (Syntax a) (Both annotation)) (Patch (Term a annotation))
 
 type instance Base (Free f a) = FreeF f a
-instance (Functor f) => Foldable.Foldable (Free f a) where project = runFree
-instance (Functor f) => Foldable.Unfoldable (Free f a) where embed = free
+instance Functor f => Foldable.Foldable (Free f a) where project = runFree
+instance Functor f => Foldable.Unfoldable (Free f a) where embed = free
 
 diffSum :: (Patch (Term a annotation) -> Integer) -> Diff a annotation -> Integer
 diffSum patchCost diff = sum $ fmap patchCost diff
@@ -22,3 +24,22 @@ diffSum patchCost diff = sum $ fmap patchCost diff
 -- | The sum of the node count of the diff’s patches.
 diffCost :: Diff a annotation -> Integer
 diffCost = diffSum $ patchSum termSize
+
+-- | Merge a diff using a function to provide the Term (in Maybe, to simplify recovery of the before/after state) for every Patch.
+mergeMaybe :: (Patch (Term leaf annotation) -> Maybe (Term leaf annotation)) -> Diff leaf annotation -> Maybe (Term leaf annotation)
+mergeMaybe transform = cata algebra . fmap transform
+  where algebra :: FreeF (CofreeF (Syntax leaf) (Both annotation)) (Maybe (Term leaf annotation)) (Maybe (Term leaf annotation)) -> Maybe (Term leaf annotation)
+        algebra (Pure p) = p
+        algebra (Free (annotations :< syntax)) = Just . cofree $ Both.fst annotations :< case syntax of
+          Leaf s -> Leaf s
+          Indexed i -> Indexed (catMaybes i)
+          Fixed i -> Fixed (catMaybes i)
+          Keyed i -> Keyed (Map.fromList (Map.toList i >>= (\ (k, v) -> maybe [] (pure . (,) k) v)))
+
+-- | Recover the before state of a diff.
+beforeTerm :: Diff leaf annotation -> Maybe (Term leaf annotation)
+beforeTerm = mergeMaybe before
+
+-- | Recover the after state of a diff.
+afterTerm :: Diff leaf annotation -> Maybe (Term leaf annotation)
+afterTerm = mergeMaybe after
diff --git a/src/Diff/Arbitrary.hs b/src/Diff/Arbitrary.hs
index f187a699e..d08397b7e 100644
--- a/src/Diff/Arbitrary.hs
+++ b/src/Diff/Arbitrary.hs
@@ -4,7 +4,6 @@ import Diff
 import Data.Bifunctor.Join
 import Data.Bifunctor.Join.Arbitrary ()
 import Data.Functor.Foldable (unfold)
-import qualified Data.List as List
 import qualified Data.OrderedMap as Map
 import Patch
 import Patch.Arbitrary ()
@@ -13,32 +12,25 @@ import Prologue
 import Term.Arbitrary
 import Test.QuickCheck hiding (Fixed)
 
-newtype ArbitraryDiff leaf annotation = ArbitraryDiff { unArbitraryDiff :: FreeF (CofreeF (Syntax leaf) (Join (,) annotation)) (Patch (ArbitraryTerm leaf annotation)) (ArbitraryDiff leaf annotation) }
+data ArbitraryDiff leaf annotation
+  = ArbitraryFree (Join (,) annotation) (Syntax leaf (ArbitraryDiff leaf annotation))
+  | ArbitraryPure (Patch (ArbitraryTerm leaf annotation))
   deriving (Show, Eq, Generic)
 
+unArbitraryDiff :: ArbitraryDiff leaf annotation -> FreeF (CofreeF (Syntax leaf) (Join (,) annotation)) (Patch (ArbitraryTerm leaf annotation)) (ArbitraryDiff leaf annotation)
+unArbitraryDiff (ArbitraryFree a s) = Free (a :< s)
+unArbitraryDiff (ArbitraryPure p) = Pure p
+
 toDiff :: ArbitraryDiff leaf annotation -> Diff leaf annotation
 toDiff = fmap (fmap toTerm) . unfold unArbitraryDiff
 
 diffOfSize :: (Arbitrary leaf, Arbitrary annotation) => Int -> Gen (ArbitraryDiff leaf annotation)
 diffOfSize n
-  | n <= 0 = (ArbitraryDiff .) . (Free .) . (:<) <$> arbitrary <*> syntaxOfSize n
+  | n <= 0 = ArbitraryFree <$> arbitrary <*> syntaxOfSize diffOfSize n
   | otherwise = oneof
-    [ (ArbitraryDiff .) . (Free .) . (:<) <$> arbitrary <*> syntaxOfSize n
-    , ArbitraryDiff . Pure <$> patchOfSize n ]
-  where syntaxOfSize n | n <= 1 = oneof $ (Leaf <$> arbitrary) : branchGeneratorsOfSize n
-                       | otherwise = oneof $ branchGeneratorsOfSize n
-        branchGeneratorsOfSize n =
-          [ Indexed <$> childrenOfSize (pred n)
-          , Fixed <$> childrenOfSize (pred n)
-          , (Keyed .) . (Map.fromList .) . zip <$> infiniteListOf arbitrary <*> childrenOfSize (pred n)
-          ]
-        childrenOfSize n | n <= 0 = pure []
-        childrenOfSize n = do
-          m <- choose (1, n)
-          first <- diffOfSize m
-          rest <- childrenOfSize (n - m)
-          pure $! first : rest
-        patchOfSize 1 = oneof [ Insert <$> termOfSize 1
+    [ ArbitraryFree <$> arbitrary <*> syntaxOfSize diffOfSize n
+    , ArbitraryPure <$> patchOfSize n ]
+  where patchOfSize 1 = oneof [ Insert <$> termOfSize 1
                               , Delete <$> termOfSize 1 ]
         patchOfSize n = do
           m <- choose (1, n - 1)
@@ -59,11 +51,4 @@ instance (Eq leaf, Eq annotation, Arbitrary leaf, Arbitrary annotation) => Arbit
     m <- choose (0, n)
     diffOfSize m
 
-  shrink diff@(ArbitraryDiff annotated) = case annotated of
-    Free (annotation :< syntax) -> (subterms diff ++) $ filter (/= diff) $
-      (ArbitraryDiff .) . (Free .) . (:<) <$> shrink annotation <*> case syntax of
-        Leaf a -> Leaf <$> shrink a
-        Indexed i -> Indexed <$> (List.subsequences i >>= recursivelyShrink)
-        Fixed f -> Fixed <$> (List.subsequences f >>= recursivelyShrink)
-        Keyed k -> Keyed . Map.fromList <$> (List.subsequences (Map.toList k) >>= recursivelyShrink)
-    Pure patch -> ArbitraryDiff . Pure <$> shrink patch
+  shrink = genericShrink
diff --git a/src/Diffing.hs b/src/Diffing.hs
index 299dc76d6..415a6444e 100644
--- a/src/Diffing.hs
+++ b/src/Diffing.hs
@@ -1,4 +1,3 @@
-{-# LANGUAGE FlexibleContexts #-}
 module Diffing where
 
 import Prologue hiding (fst, snd)
diff --git a/src/Info.hs b/src/Info.hs
index 7c459f18a..cd2d3d849 100644
--- a/src/Info.hs
+++ b/src/Info.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE DataKinds, FlexibleContexts, GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE DataKinds, GeneralizedNewtypeDeriving #-}
 module Info where
 
 import Data.Record
@@ -7,9 +7,9 @@ import Category
 import Range
 
 newtype Size = Size { unSize :: Integer }
-  deriving (Eq, Num, Show)
+  deriving (Eq, Num, Ord, Show)
 newtype Cost = Cost { unCost :: Integer }
-  deriving (Eq, Num, Show)
+  deriving (Eq, Num, Ord, Show)
 
 type InfoFields = '[ Range, Category, Size, Cost ]
 
diff --git a/src/Interpreter.hs b/src/Interpreter.hs
index bc1aec57b..da3fc4423 100644
--- a/src/Interpreter.hs
+++ b/src/Interpreter.hs
@@ -1,13 +1,20 @@
 module Interpreter (Comparable, DiffConstructor, diffTerms) where
 
 import Algorithm
+import Category
+import Data.Align
 import Data.Functor.Foldable
 import Data.Functor.Both
-import qualified Data.OrderedMap as Map
-import qualified Data.List as List
+import Data.Hashable
 import Data.List ((\\))
+import qualified Data.List as List
 import Data.OrderedMap ((!))
+import qualified Data.OrderedMap as Map
+import Data.RandomWalkSimilarity
+import Data.Record
+import Data.These
 import Diff
+import Info
 import Operation
 import Patch
 import Prologue hiding (lookup)
@@ -16,17 +23,17 @@ import Syntax
 import Term
 
 -- | Returns whether two terms are comparable
-type Comparable a annotation = Term a annotation -> Term a annotation -> Bool
+type Comparable leaf annotation = Term leaf annotation -> Term leaf annotation -> Bool
 
 -- | Constructs a diff from the CofreeF containing its annotation and syntax. This function has the opportunity to, for example, cache properties in the annotation.
 type DiffConstructor leaf annotation = CofreeF (Syntax leaf) (Both annotation) (Diff leaf annotation) -> Diff leaf annotation
 
 -- | Diff two terms, given a function that determines whether two terms can be compared and a cost function.
-diffTerms :: (Eq a, Eq annotation) => DiffConstructor a annotation -> Comparable a annotation -> Cost (Diff a annotation) -> Term a annotation -> Term a annotation -> Diff a annotation
+diffTerms :: (Eq leaf, Hashable leaf, Ord (Record fields), HasField fields Category) => DiffConstructor leaf (Record fields) -> Comparable leaf (Record fields) -> SES.Cost (Diff leaf (Record fields)) -> Term leaf (Record fields) -> Term leaf (Record fields) -> Diff leaf (Record fields)
 diffTerms construct comparable cost a b = fromMaybe (pure $ Replace a b) $ constructAndRun construct comparable cost a b
 
 -- | Constructs an algorithm and runs it
-constructAndRun :: (Eq a, Eq annotation) => DiffConstructor a annotation -> Comparable a annotation -> Cost (Diff a annotation) -> Term a annotation -> Term a annotation -> Maybe (Diff a annotation)
+constructAndRun :: (Eq leaf, Hashable leaf, Ord (Record fields), HasField fields Category) => DiffConstructor leaf (Record fields) -> Comparable leaf (Record fields) -> SES.Cost (Diff leaf (Record fields)) -> Term leaf (Record fields) -> Term leaf (Record fields) -> Maybe (Diff leaf (Record fields))
 constructAndRun _ comparable _ a b | not $ comparable a b = Nothing
 
 constructAndRun construct _ _ a b | (() <$ a) == (() <$ b) = hylo construct runCofree <$> zipTerms a b
@@ -41,21 +48,23 @@ constructAndRun construct comparable cost t1 t2 =
     annotate = pure . construct . (both annotation1 annotation2 :<)
 
 -- | Runs the diff algorithm
-run :: (Eq a, Eq annotation) => DiffConstructor a annotation -> Comparable a annotation -> Cost (Diff a annotation) -> Algorithm a annotation (Diff a annotation) -> Maybe (Diff a annotation)
+run :: (Eq leaf, Hashable leaf, Ord (Record fields), HasField fields Category) => DiffConstructor leaf (Record fields) -> Comparable leaf (Record fields) -> SES.Cost (Diff leaf (Record fields)) -> Algorithm leaf (Record fields) (Diff leaf (Record fields)) -> Maybe (Diff leaf (Record fields))
 run construct comparable cost algorithm = case runFree algorithm of
   Pure diff -> Just diff
   Free (Recursive t1 t2 f) -> run construct comparable cost . f $ recur a b where
     (annotation1 :< a, annotation2 :< b) = (runCofree t1, runCofree t2)
     annotate = construct . (both annotation1 annotation2 :<)
 
-    recur (Indexed a') (Indexed b') | length a' == length b' = annotate . Indexed $ zipWith (diffTerms construct comparable cost) a' b'
-    recur (Fixed a') (Fixed b') | length a' == length b' = annotate . Fixed $ zipWith (diffTerms construct comparable cost) a' b'
+    recur (Indexed a') (Indexed b') = annotate . Indexed $ alignWith diffThese a' b'
+    recur (Fixed a') (Fixed b') = annotate . Fixed $ alignWith diffThese a' b'
     recur (Keyed a') (Keyed b') | Map.keys a' == bKeys = annotate . Keyed . Map.fromList . fmap repack $ bKeys where
       bKeys = Map.keys b'
       repack key = (key, interpretInBoth key a' b')
       interpretInBoth key x y = diffTerms construct comparable cost (x ! key) (y ! key)
     recur _ _ = pure $ Replace (cofree (annotation1 :< a)) (cofree (annotation2 :< b))
 
+    diffThese = these (pure . Delete) (pure . Insert) (diffTerms construct comparable cost)
+
   Free (ByKey a b f) -> run construct comparable cost $ f byKey where
     byKey = Map.fromList $ toKeyValue <$> List.union aKeys bKeys
     toKeyValue key | key `List.elem` deleted = (key, pure . Delete $ a ! key)
@@ -67,3 +76,6 @@ run construct comparable cost algorithm = case runFree algorithm of
     inserted = bKeys \\ aKeys
 
   Free (ByIndex a b f) -> run construct comparable cost . f $ ses (constructAndRun construct comparable cost) cost a b
+
+  Free (ByRandomWalkSimilarity a b f) -> run construct comparable cost . f $ rws (constructAndRun construct comparable cost) getLabel a b
+    where getLabel = category
diff --git a/src/Operation.hs b/src/Operation.hs
index 32e553842..b920748b6 100644
--- a/src/Operation.hs
+++ b/src/Operation.hs
@@ -10,11 +10,11 @@ data Operation
   a -- ^ The type of leaves in the syntax tree, typically String, but possibly some datatype representing different leaves more precisely.
   annotation -- ^ The type of annotations.
   f -- ^ The type representing another level of the diffing algorithm. Often Algorithm.
-  =
   -- | Recursively diff two terms and pass the result to the continuation.
-  Recursive (Term a annotation) (Term a annotation) (Diff a annotation -> f)
+  = Recursive (Term a annotation) (Term a annotation) (Diff a annotation -> f)
   -- | Diff two dictionaries and pass the result to the continuation.
   | ByKey (OrderedMap Text (Term a annotation)) (OrderedMap Text (Term a annotation)) (OrderedMap Text (Diff a annotation) -> f)
   -- | Diff two arrays and pass the result to the continuation.
   | ByIndex [Term a annotation] [Term a annotation] ([Diff a annotation] -> f)
+  | ByRandomWalkSimilarity [Term a annotation] [Term a annotation] ([Diff a annotation] -> f)
   deriving Functor
diff --git a/src/Parser.hs b/src/Parser.hs
index 44a9a9114..9e04e07e5 100644
--- a/src/Parser.hs
+++ b/src/Parser.hs
@@ -1,4 +1,3 @@
-{-# LANGUAGE FlexibleContexts #-}
 module Parser where
 
 import Prologue hiding (Constructor)
diff --git a/src/Patch.hs b/src/Patch.hs
index d5e38583c..d1500119c 100644
--- a/src/Patch.hs
+++ b/src/Patch.hs
@@ -12,19 +12,19 @@ import Data.These
 import Prologue
 
 -- | An operation to replace, insert, or delete an item.
-data Patch a =
-  Replace a a
+data Patch a
+  = Replace a a
   | Insert a
   | Delete a
-  deriving (Eq, Foldable, Functor, Show, Traversable)
+  deriving (Eq, Foldable, Functor, Ord, Show, Traversable)
 
 -- | Return the item from the after side of the patch.
 after :: Patch a -> Maybe a
-after = maybeFst . unPatch
+after = maybeSnd . unPatch
 
 -- | Return the item from the before side of the patch.
 before :: Patch a -> Maybe a
-before = maybeSnd . unPatch
+before = maybeFst . unPatch
 
 -- | Return both sides of a patch.
 unPatch :: Patch a -> These a a
diff --git a/src/SplitDiff.hs b/src/SplitDiff.hs
index 30011fde6..9dc6459f7 100644
--- a/src/SplitDiff.hs
+++ b/src/SplitDiff.hs
@@ -1,4 +1,3 @@
-{-# LANGUAGE FlexibleContexts #-}
 module SplitDiff where
 
 import Data.Record
diff --git a/src/Syntax.hs b/src/Syntax.hs
index 80657ea46..a7168cfd4 100644
--- a/src/Syntax.hs
+++ b/src/Syntax.hs
@@ -1,8 +1,10 @@
 module Syntax where
 
 import Prologue
-import Data.OrderedMap
+import Data.OrderedMap as Map
+import Data.Text.Arbitrary ()
 import qualified Data.Text as T
+import Test.QuickCheck hiding (Fixed)
 
 -- | A node in an abstract syntax tree.
 data Syntax
@@ -17,4 +19,27 @@ data Syntax
   | Fixed [f]
   -- | A branch of child nodes indexed by some String identity. This is useful for identifying e.g. methods & properties in a class scope by their names. Note that comments can generally occur in these scopes as well; one strategy for dealing with this is to identify comments by their text in the source.
   | Keyed (OrderedMap T.Text f)
-  deriving (Functor, Show, Eq, Foldable, Traversable)
+  deriving (Eq, Foldable, Functor, Generic, Ord, Show, Traversable)
+
+
+-- Instances
+
+syntaxOfSize :: Arbitrary leaf => (Int -> Gen f) -> Int -> Gen (Syntax leaf f)
+syntaxOfSize recur n | n <= 1 = oneof $ (Leaf <$> arbitrary) : branchGeneratorsOfSize n
+                     | otherwise = oneof $ branchGeneratorsOfSize n
+  where branchGeneratorsOfSize n =
+          [ Indexed <$> childrenOfSize (pred n)
+          , Fixed <$> childrenOfSize (pred n)
+          , (Keyed .) . (Map.fromList .) . zip <$> infiniteListOf arbitrary <*> childrenOfSize (pred n)
+          ]
+        childrenOfSize n | n <= 0 = pure []
+        childrenOfSize n = do
+          m <- choose (1, n)
+          first <- recur m
+          rest <- childrenOfSize (n - m)
+          pure $! first : rest
+
+instance (Arbitrary leaf, Arbitrary f) => Arbitrary (Syntax leaf f) where
+  arbitrary = sized (syntaxOfSize (`resize` arbitrary) )
+
+  shrink = genericShrink
diff --git a/src/Term.hs b/src/Term.hs
index 1f4b915f8..c76207f10 100644
--- a/src/Term.hs
+++ b/src/Term.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE TypeFamilies, TypeSynonymInstances #-}
+{-# OPTIONS_GHC -fno-warn-orphans #-}
 module Term where
 
 import Prologue
diff --git a/src/Term/Arbitrary.hs b/src/Term/Arbitrary.hs
index 2ffc28fb1..2c0921da4 100644
--- a/src/Term/Arbitrary.hs
+++ b/src/Term/Arbitrary.hs
@@ -2,52 +2,36 @@
 module Term.Arbitrary where
 
 import Data.Functor.Foldable (Base, cata, unfold, Unfoldable(embed))
-import qualified Data.List as List
-import qualified Data.OrderedMap as Map
 import Data.Text.Arbitrary ()
 import Prologue
 import Syntax
 import Term
 import Test.QuickCheck hiding (Fixed)
 
-newtype ArbitraryTerm leaf annotation = ArbitraryTerm { unArbitraryTerm :: TermF leaf annotation (ArbitraryTerm leaf annotation) }
+data ArbitraryTerm leaf annotation = ArbitraryTerm annotation (Syntax leaf (ArbitraryTerm leaf annotation))
   deriving (Show, Eq, Generic)
 
+unArbitraryTerm :: ArbitraryTerm leaf annotation -> TermF leaf annotation (ArbitraryTerm leaf annotation)
+unArbitraryTerm (ArbitraryTerm a s) = a :< s
+
 toTerm :: ArbitraryTerm leaf annotation -> Term leaf annotation
 toTerm = unfold unArbitraryTerm
 
 termOfSize :: (Arbitrary leaf, Arbitrary annotation) => Int -> Gen (ArbitraryTerm leaf annotation)
-termOfSize n = (ArbitraryTerm .) . (:<) <$> arbitrary <*> syntaxOfSize n
-  where syntaxOfSize n | n <= 1 = oneof $ (Leaf <$> arbitrary) : branchGeneratorsOfSize n
-                       | otherwise = oneof $ branchGeneratorsOfSize n
-        branchGeneratorsOfSize n =
-          [ Indexed <$> childrenOfSize (pred n)
-          , Fixed <$> childrenOfSize (pred n)
-          , (Keyed .) . (Map.fromList .) . zip <$> infiniteListOf arbitrary <*> childrenOfSize (pred n)
-          ]
-        childrenOfSize n | n <= 0 = pure []
-        childrenOfSize n = do
-          m <- choose (1, n)
-          first <- termOfSize m
-          rest <- childrenOfSize (n - m)
-          pure $! first : rest
+termOfSize n = ArbitraryTerm <$> arbitrary <*> syntaxOfSize termOfSize n
 
 arbitraryTermSize :: ArbitraryTerm leaf annotation -> Int
 arbitraryTermSize = cata (succ . sum) . toTerm
 
+
 -- Instances
 
-type instance Base (ArbitraryTerm leaf annotation) = CofreeF (Syntax leaf) annotation
-instance Unfoldable (ArbitraryTerm leaf annotation) where embed = ArbitraryTerm
+type instance Base (ArbitraryTerm leaf annotation) = TermF leaf annotation
+instance Unfoldable (ArbitraryTerm leaf annotation) where embed (a :< s) = ArbitraryTerm a s
 
 instance (Eq leaf, Eq annotation, Arbitrary leaf, Arbitrary annotation) => Arbitrary (ArbitraryTerm leaf annotation) where
   arbitrary = sized $ \ n -> do
     m <- choose (0, n)
     termOfSize m
 
-  shrink term@(ArbitraryTerm (annotation :< syntax)) = (subterms term ++) $ filter (/= term) $
-    (ArbitraryTerm .) . (:<) <$> shrink annotation <*> case syntax of
-      Leaf a -> Leaf <$> shrink a
-      Indexed i -> Indexed <$> (List.subsequences i >>= recursivelyShrink)
-      Fixed f -> Fixed <$> (List.subsequences f >>= recursivelyShrink)
-      Keyed k -> Keyed . Map.fromList <$> (List.subsequences (Map.toList k) >>= recursivelyShrink)
+  shrink = genericShrink
diff --git a/test/AlignmentSpec.hs b/test/AlignmentSpec.hs
index 1c8757f0e..13730d8a2 100644
--- a/test/AlignmentSpec.hs
+++ b/test/AlignmentSpec.hs
@@ -1,7 +1,6 @@
 module AlignmentSpec where
 
 import Alignment
-import ArbitraryTerm ()
 import Control.Arrow ((&&&))
 import Control.Monad.State
 import Data.Align hiding (align)
@@ -49,7 +48,7 @@ spec = parallel $ do
 
     prop "covers every input line" $
       \ elements -> let (_, children, ranges) = toAlignBranchInputs elements in
-        join <$> (traverse (modifyJoin (fromThese [] []) . fmap pure . fmap Prologue.fst) (alignBranch Prologue.snd children ranges)) `shouldBe` ranges
+        join <$> (traverse (modifyJoin (fromThese [] []) . fmap (pure . Prologue.fst)) (alignBranch Prologue.snd children ranges)) `shouldBe` ranges
 
     prop "covers every input child" $
       \ elements -> let (_, children, ranges) = toAlignBranchInputs elements in
@@ -194,7 +193,7 @@ spec = parallel $ do
                       (info 8 9 `branch` [ info 8 9 `leaf` "c" ]))
         ]
 
-  describe "numberedRows" $
+  describe "numberedRows" $ do
     prop "counts only non-empty values" $
       \ xs -> counts (numberedRows (xs :: [Join These Char])) `shouldBe` length . catMaybes <$> Join (unalign (runJoin <$> xs))
 
@@ -237,9 +236,9 @@ keysOfAlignedChildren lines = lines >>= these identity identity (++) . runJoin .
 instance Arbitrary BranchElement where
   arbitrary = oneof [ key >>= \ key -> Child key <$> joinTheseOf (contents key)
                     , Margin <$> joinTheseOf margin ]
-    where key = listOf1 (elements (['a'..'z'] ++ ['A'..'Z'] ++ ['0'..'9']))
+    where key = listOf1 (elements (['a'..'z'] <> ['A'..'Z'] <> ['0'..'9']))
           contents key = wrap key <$> listOf (padding '*')
-          wrap key contents = "(" ++ key ++ contents ++ ")" :: String
+          wrap key contents = "(" <> key <> contents <> ")" :: String
           margin = listOf (padding '-')
           padding char = frequency [ (10, pure char)
                                    , (1, pure '\n') ]
diff --git a/test/ArbitraryTerm.hs b/test/ArbitraryTerm.hs
deleted file mode 100644
index 5590f1749..000000000
--- a/test/ArbitraryTerm.hs
+++ /dev/null
@@ -1,16 +0,0 @@
-{-# OPTIONS_GHC -fno-warn-orphans #-}
-module ArbitraryTerm where
-
-import Data.Text.Arbitrary ()
-import Data.These.Arbitrary ()
-import Prologue hiding (fst, snd)
-import Source hiding ((++))
-import Test.QuickCheck hiding (Fixed)
-
-data CategorySet = A | B | C | D deriving (Eq, Show)
-
-instance Arbitrary CategorySet where
-  arbitrary = elements [ A, B, C, D ]
-
-instance Arbitrary a => Arbitrary (Source a) where
-  arbitrary = Source.fromList <$> arbitrary
diff --git a/test/Data/RandomWalkSimilarity/Spec.hs b/test/Data/RandomWalkSimilarity/Spec.hs
new file mode 100644
index 000000000..cdecd4a25
--- /dev/null
+++ b/test/Data/RandomWalkSimilarity/Spec.hs
@@ -0,0 +1,39 @@
+module Data.RandomWalkSimilarity.Spec where
+
+import Category
+import Data.DList as DList hiding (toList)
+import Data.RandomWalkSimilarity
+import qualified Data.Set as Set
+import Diff
+import Patch
+import Prologue
+import Syntax
+import Term
+import Term.Arbitrary
+import Test.Hspec
+import Test.Hspec.QuickCheck
+import Test.QuickCheck
+
+spec :: Spec
+spec = parallel $ do
+  describe "pqGrams" $ do
+    prop "produces grams with stems of the specified length" . forAll (arbitrary `suchThat` (\ (_, p, q) -> p > 0 && q > 0)) $
+      \ (term, p, q) -> pqGrams p q identity (toTerm term :: Term Text Text) `shouldSatisfy` all ((== p) . length . stem)
+
+    prop "produces grams with bases of the specified length" . forAll (arbitrary `suchThat` (\ (_, p, q) -> p > 0 && q > 0)) $
+      \ (term, p, q) -> pqGrams p q identity (toTerm term :: Term Text Text) `shouldSatisfy` all ((== q) . length . base)
+
+  describe "featureVector" $ do
+    prop "produces a vector of the specified dimension" . forAll (arbitrary `suchThat` ((> 0) . Prologue.snd)) $
+      \ (grams, d) -> length (featureVector d (fromList (grams :: [Gram Text]))) `shouldBe` d
+
+  describe "rws" $ do
+    let compare a b = if extract a == extract b then Just (pure (Replace a b)) else Nothing
+    prop "produces correct diffs" . forAll (scale (`div` 4) arbitrary) $
+      \ (as, bs) -> let tas = toTerm <$> as
+                        tbs = toTerm <$> bs
+                        diff = free (Free (pure Program :< Indexed (rws compare identity tas tbs :: [Diff Text Category]))) in
+        (childrenOf <$> beforeTerm diff, childrenOf <$> afterTerm diff) `shouldBe` (Just (Set.fromList tas), Just (Set.fromList tbs))
+
+childrenOf :: (Ord leaf, Ord annotation) => Term leaf annotation -> Set.Set (Term leaf annotation)
+childrenOf = Set.fromList . toList . unwrap
diff --git a/test/Diff/Spec.hs b/test/Diff/Spec.hs
new file mode 100644
index 000000000..38e2b536a
--- /dev/null
+++ b/test/Diff/Spec.hs
@@ -0,0 +1,38 @@
+{-# LANGUAGE DataKinds #-}
+module Diff.Spec where
+
+import Category
+import Data.Record
+import Data.Text.Arbitrary ()
+import Diff
+import Diff.Arbitrary
+import Interpreter
+import Prologue
+import Term.Arbitrary
+import Test.Hspec
+import Test.Hspec.QuickCheck
+import Test.QuickCheck
+
+spec :: Spec
+spec = parallel $ do
+  prop "equality is reflexive" $
+    \ a b -> let diff = diffTerms (free . Free) (==) diffCost (toTerm a) (toTerm (b :: ArbitraryTerm Text (Record '[Category]))) in
+      diff `shouldBe` diff
+
+  prop "equal terms produce identity diffs" $
+    \ a -> let term = toTerm (a :: ArbitraryTerm Text (Record '[Category])) in
+      diffCost (diffTerms (free . Free) (==) diffCost term term) `shouldBe` 0
+
+  describe "beforeTerm" $ do
+    prop "recovers the before term" $
+      \ a b -> let diff = diffTerms (free . Free) (==) diffCost (toTerm a) (toTerm (b :: ArbitraryTerm Text (Record '[Category]))) in
+        beforeTerm diff `shouldBe` Just (toTerm a)
+
+  describe "afterTerm" $ do
+    prop "recovers the after term" $
+      \ a b -> let diff = diffTerms (free . Free) (==) diffCost (toTerm a) (toTerm (b :: ArbitraryTerm Text (Record '[Category]))) in
+        afterTerm diff `shouldBe` Just (toTerm b)
+
+  describe "ArbitraryDiff" $ do
+    prop "generates diffs of a specific size" . forAll ((arbitrary >>= \ n -> (,) n <$> diffOfSize n) `suchThat` ((> 0) . fst)) $
+      \ (n, diff) -> arbitraryDiffSize (diff :: ArbitraryDiff Text ()) `shouldBe` n
diff --git a/test/InterpreterSpec.hs b/test/InterpreterSpec.hs
index fb8422cee..0b62d9790 100644
--- a/test/InterpreterSpec.hs
+++ b/test/InterpreterSpec.hs
@@ -1,23 +1,30 @@
+{-# LANGUAGE DataKinds #-}
 module InterpreterSpec where
 
-import Prologue
+import Category
 import Diff
 import Data.Record
-import qualified Interpreter as I
-import Range
-import Syntax
+import Interpreter
 import Patch
-import Info
-import Category
+import Prologue
+import Syntax
+import Term.Arbitrary
 import Test.Hspec
+import Test.Hspec.QuickCheck
 
 spec :: Spec
-spec = parallel $
-  describe "interpret" $
+spec = parallel $ do
+  describe "interpret" $ do
     it "returns a replacement when comparing two unicode equivalent terms" $
-      I.diffTerms (free . Free) ((==) `on` extract) diffCost (cofree ((range .: StringLiteral .: 0 .: 0 .: RNil) :< Leaf "t\776")) (cofree ((range2 .: StringLiteral .: 0 .: 0 .: RNil) :< Leaf "\7831")) `shouldBe`
-      free (Pure (Replace (cofree ((range .: StringLiteral .: 0 .: 0 .: RNil) :< Leaf "t\776")) (cofree ((range2 .: StringLiteral .: 0 .: 0 .: RNil) :< Leaf "\7831"))))
+      let termA = cofree $ (StringLiteral .: RNil) :< Leaf ("t\776" :: Text)
+          termB = cofree $ (StringLiteral .: RNil) :< Leaf "\7831" in
+          diffTerms (free . Free) ((==) `on` extract) diffCost termA termB `shouldBe` free (Pure (Replace termA termB))
 
-    where
-      range = Range 0 2
-      range2 = Range 0 1
+    prop "produces correct diffs" $
+      \ a b -> let diff = diffTerms (free . Free) ((==) `on` extract) diffCost (toTerm a) (toTerm b) :: Diff Text (Record '[Category]) in
+                   (beforeTerm diff, afterTerm diff) `shouldBe` (Just (toTerm a), Just (toTerm b))
+
+    prop "constructs zero-cost diffs of equal terms" $
+      \ a -> let term = toTerm a
+                 diff = diffTerms (free . Free) ((==) `on` extract) diffCost term term :: Diff Text (Record '[Category]) in
+                 diffCost diff `shouldBe` 0
diff --git a/test/Spec.hs b/test/Spec.hs
index 035688e32..e38ac01e2 100644
--- a/test/Spec.hs
+++ b/test/Spec.hs
@@ -3,19 +3,23 @@ module Main where
 import Prologue
 import qualified AlignmentSpec
 import qualified CorpusSpec
+import qualified Data.RandomWalkSimilarity.Spec
+import qualified Diff.Spec
+import qualified DiffSummarySpec
 import qualified InterpreterSpec
 import qualified OrderedMapSpec
 import qualified PatchOutputSpec
 import qualified TermSpec
-import qualified DiffSummarySpec
 import Test.Hspec
 
 main :: IO ()
-main = hspec $ parallel $ do
+main = hspec . parallel $ do
   describe "Alignment" AlignmentSpec.spec
   describe "Corpus" CorpusSpec.spec
+  describe "Data.RandomWalkSimilarity" Data.RandomWalkSimilarity.Spec.spec
+  describe "Diff.Spec" Diff.Spec.spec
+  describe "DiffSummary" DiffSummarySpec.spec
   describe "Interpreter" InterpreterSpec.spec
   describe "OrderedMap" OrderedMapSpec.spec
   describe "PatchOutput" PatchOutputSpec.spec
   describe "Term" TermSpec.spec
-  describe "DiffSummary" DiffSummarySpec.spec
diff --git a/test/TermSpec.hs b/test/TermSpec.hs
index 42a1f9c28..f0a209557 100644
--- a/test/TermSpec.hs
+++ b/test/TermSpec.hs
@@ -1,11 +1,6 @@
 module TermSpec where
 
-import ArbitraryTerm
-import Data.String
 import Data.Text.Arbitrary ()
-import Diff
-import Diff.Arbitrary
-import Interpreter
 import Prologue
 import Term.Arbitrary
 import Test.Hspec
@@ -16,21 +11,8 @@ spec :: Spec
 spec = parallel $ do
   describe "Term" $ do
     prop "equality is reflexive" $
-      \ a -> toTerm a == toTerm (a :: ArbitraryTerm String ())
+      \ a -> toTerm a `shouldBe` toTerm (a :: ArbitraryTerm Text ())
 
-  describe "ArbitraryTerm" $
-    prop "generates terms of a specific size" $ forAll ((arbitrary >>= \ n -> (,) n <$> termOfSize n) `suchThat` ((> 0) . fst)) $
-      \ (n, term) -> arbitraryTermSize (term :: ArbitraryTerm String ()) `shouldBe` n
-
-  describe "ArbitraryDiff" $
-    prop "generates diffs of a specific size" $ forAll ((arbitrary >>= \ n -> (,) n <$> diffOfSize n) `suchThat` ((> 0) . fst)) $
-      \ (n, diff) -> arbitraryDiffSize (diff :: ArbitraryDiff String ()) `shouldBe` n
-
-  describe "Diff" $ do
-    prop "equality is reflexive" $
-      \ a b -> let diff = diffTerms (free . Free) (==) diffCost (toTerm a) (toTerm (b :: ArbitraryTerm String CategorySet)) in
-        diff == diff
-
-    prop "equal terms produce identity diffs" $
-      \ a -> let term = toTerm (a :: ArbitraryTerm String CategorySet) in
-        diffCost (diffTerms (free . Free) (==) diffCost term term) == 0
+  describe "ArbitraryTerm" $ do
+    prop "generates terms of a specific size" . forAll ((arbitrary >>= \ n -> (,) n <$> termOfSize n) `suchThat` ((> 0) . fst)) $
+      \ (n, term) -> arbitraryTermSize (term :: ArbitraryTerm Text ()) `shouldBe` n