diff --git a/src/RWS.hs b/src/RWS.hs
index 1363f05a3..f80369e6e 100644
--- a/src/RWS.hs
+++ b/src/RWS.hs
@@ -20,23 +20,68 @@ import Data.Functor.Classes.Eq.Generic
 --     -> [Term f (Record fields)] -- ^ The list of new terms.
 --     -> [These (Term f (Record fields)) (Term f (Record fields))] -- ^ The resulting list of similarity-matched diffs.
 -- rws editDistance canCompare as bs = undefined
+-- | A term which has not yet been mapped by `rws`, along with its feature vector summary & index.
+data UnmappedTerm f fields = UnmappedTerm {
+    termIndex :: Int -- ^ The index of the term within its root term.
+  , feature   :: FeatureVector -- ^ Feature vector
+  , term      :: Term f (Record fields) -- ^ The unmapped term
+}
+
+-- | Either a `term`, an index of a matched term, or nil.
+data TermOrIndexOrNone term = Term term | Index Int | None
+
 
 data RWS f (fields :: [*]) result where
   -- RWS :: RWS a b (EditScript a b)
   SES :: RWS f fields (RWSEditScript f fields)
+  -- FindNearestNeighbourToDiff :: TermOrIndexOrNone (UnmappedTerm f fields) ->
+  GenFeaturizedTermsAndDiffs :: HasField fields (Maybe FeatureVector) => RWSEditScript f fields -> RWS f fields ([UnmappedTerm f fields], [UnmappedTerm f fields], [(These Int Int, These (Term f (Record fields)) (Term f (Record fields)))], [TermOrIndexOrNone (UnmappedTerm f fields)])
   -- EraseFeatureVector :: forall a b f fields. RwsF a b (EditScript (Term f (Record fields)) (Term f (Record fields)))
 
-
 type FeatureVector = Array Int Double
 
 type RWSEditScript f fields = [These (Term f (Record fields)) (Term f (Record fields))]
 
-run :: (Eq1 f, Functor f, HasField fields Category, Foldable t) => t (Term f (Record fields)) -> t (Term f (Record fields)) -> Eff '[RWS f fields] (RWSEditScript f fields) -> RWSEditScript f fields
+run :: (Eq1 f, Functor f, HasField fields Category, HasField fields FeatureVector, Foldable t) => t (Term f (Record fields)) -> t (Term f (Record fields)) -> Eff '[RWS f fields] (RWSEditScript f fields) -> RWSEditScript f fields
 run _ _ (Val x) = x
 run as bs (E u q) = case decompose u of
   Right SES ->
     let sesDiffs = ses (gliftEq (==) `on` fmap category) as bs in
       RWS.run as bs (apply q sesDiffs)
+  Right (GenFeaturizedTermsAndDiffs sesDiffs) -> RWS.run as bs . apply q $ evalState (genFeaturizedTermsAndDiffs sesDiffs) (0, 0)
+
+genFeaturizedTermsAndDiffs :: (Functor f, HasField fields (Maybe FeatureVector)) => RWSEditScript f fields -> State (Int, Int) ([UnmappedTerm f fields], [UnmappedTerm f fields], [(These Int Int, These (Term f (Record fields)) (Term f (Record fields)))], [TermOrIndexOrNone (UnmappedTerm f fields)])
+genFeaturizedTermsAndDiffs sesDiffs = case sesDiffs of
+  [] -> pure ([], [], [], [])
+  (diff : diffs) -> do
+    (counterA, counterB) <- get
+    case diff of
+      This term -> do
+        put (succ counterA, counterB)
+        (as, bs, mappedDiffs, allDiffs) <- genFeaturizedTermsAndDiffs diffs
+        pure (as <> pure (featurize counterA term), bs, mappedDiffs, allDiffs <> pure None)
+      That term -> do
+        put (counterA, succ counterB)
+        (as, bs, mappedDiffs, allDiffs) <- genFeaturizedTermsAndDiffs diffs
+        pure (as, bs <> pure (featurize counterB term), mappedDiffs, allDiffs <> pure (Term (featurize counterB term)))
+      These a b -> do
+        put (succ counterA, succ counterB)
+        (as, bs, mappedDiffs, allDiffs) <- genFeaturizedTermsAndDiffs diffs
+        pure (as, bs, mappedDiffs <> pure (These counterA counterB, These a b), allDiffs <> pure (Index counterA))
+
+featurize :: (HasField fields (Maybe FeatureVector), Functor f) => Int -> Term f (Record fields) -> UnmappedTerm f fields
+featurize index term = UnmappedTerm index (let Just v = getField (extract term) in v) (eraseFeatureVector term)
+
+eraseFeatureVector :: (Functor f, HasField fields (Maybe FeatureVector)) => Term f (Record fields) -> Term f (Record fields)
+eraseFeatureVector term = let record :< functor = runCofree term in
+  cofree (setFeatureVector record Nothing :< functor)
+
+setFeatureVector :: HasField fields (Maybe FeatureVector) => Record fields -> Maybe FeatureVector -> Record fields
+setFeatureVector = setField
+
+
+
+
 
 data EditGraph a b = EditGraph { as :: !(Array Int a), bs :: !(Array Int b) }
   deriving (Eq, Show)