diff --git a/byg/src/DependencyGenerator.hs b/byg/src/DependencyGenerator.hs
index 4a92f16..cb402d5 100644
--- a/byg/src/DependencyGenerator.hs
+++ b/byg/src/DependencyGenerator.hs
@@ -41,10 +41,10 @@ import Types.Values
 import Types.Value (Valuable(..))
 import Types.Function (Function(..))
 import Types.FunctionIO (FunctionIO(..))
-import Types.Dependency (Action(..), Dependency, makeDependency)
+import Types.Dependency (Action(..), Dependency, makeDependency, makeUToken)
 
 import Control.Monad.State (MonadState, State, runState, put, get)
-import Control.Monad.Writer (MonadWriter, WriterT, execWriterT, tell)
+import Control.Monad.Writer (MonadWriter, WriterT, runWriterT, tell)
 import Data.Text (Text)
 
 newtype DepGenM a = DepGenM { unDepGenM :: WriterT [Dependency] (State Int) a }
@@ -52,21 +52,26 @@ newtype DepGenM a = DepGenM { unDepGenM :: WriterT [Dependency] (State Int) a }
 
 type DepGenM' a = DepGenM (Token a)
 
-evalDepGenM' :: Int -> DepGenM () -> ([Dependency], Int)
-evalDepGenM' top m = runState (execWriterT (unDepGenM m)) top
+runDepGenM :: Int -> DepGenM a -> ((a, [Dependency]), Int)
+runDepGenM top m = runState (runWriterT (unDepGenM m)) top
 
 evalDepGenM :: DepGenM () -> [Dependency]
-evalDepGenM m = fst (evalDepGenM' 0 m)
+evalDepGenM = snd . fst . runDepGenM 0
 
 tellDep :: Dependency -> DepGenM ()
 tellDep dep = tell [dep]
 
-genDependencyM :: (Token a -> DepGenM Dependency) -> DepGenM' a
-genDependencyM f = do
+newToken :: DepGenM' a
+newToken = do
   top <- get
   let top' = top + 1
       target = Token top'
   put top'
+  pure target
+
+genDependencyM :: (Token a -> DepGenM Dependency) -> DepGenM' a
+genDependencyM f = do
+  target <- newToken
   result <- f target
   tellDep result
   pure target
@@ -77,13 +82,6 @@ genDependency f = genDependencyM (pure . f)
 inject :: Valuable a => a -> DepGenM' a
 inject x = genDependency (makeDependency NoToken (Inject (toValue x)))
 
-getListElem :: Token [a] -> DepGenM' a
-getListElem outer = genDependency (makeDependency outer GetListElem)
-
-setListElem :: Token a -> Token [a] -> DepGenM ()
-setListElem a outer = do
-  tellDep (makeDependency a SetListElem outer)
-
 runFunction :: Function -> Token a -> DepGenM' b
 runFunction f input = genDependency (makeDependency input (Function f))
 
@@ -98,12 +96,12 @@ mapDepGenM f input = do
   input' <- toToken input
   genDependencyM $ \target -> do
     top <- get
-    let (res, top') = evalDepGenM' top $ do
-          inp <- getListElem input'
+    let (((innerInp, innerOutp), subDeps), top') = runDepGenM top $ do
+          inp <- newToken
           outp <- f inp
-          setListElem outp target
+          pure (inp, outp)
     put top'
-    pure (makeDependency input' (MapComp res) target)
+    pure (makeDependency input' (MapComp subDeps (makeUToken innerInp) (makeUToken innerOutp)) target)
 
 mapDepGenM_ :: TokenableTo [a] v => (Token a -> DepGenM ()) -> v -> DepGenM ()
 mapDepGenM_ f input = do
diff --git a/byg/src/DependencyRunner.hs b/byg/src/DependencyRunner.hs
index 6b759f3..68e59aa 100644
--- a/byg/src/DependencyRunner.hs
+++ b/byg/src/DependencyRunner.hs
@@ -66,10 +66,6 @@ runAction action input = case action of
         pure $ List $ map fst $ filter (fromValue . snd) $ zip vs mask
       _ ->
         error "unexpected"
-  GetListElem ->
-    undefined
-  SetListElem ->
-    undefined
   UntupleFst ->
     case input of
       Tuple (v, _) ->
@@ -94,5 +90,5 @@ runAction action input = case action of
         List <$> mapM (runAction UntupleSnd) vs
       _ ->
         error "unexpected"
-  MapComp subDeps ->
+  MapComp subDeps innerInput innerOutput ->
     undefined
diff --git a/byg/src/Types/Dependency.hs b/byg/src/Types/Dependency.hs
index f1c1e51..d7c527f 100644
--- a/byg/src/Types/Dependency.hs
+++ b/byg/src/Types/Dependency.hs
@@ -4,6 +4,7 @@ module Types.Dependency
   , UToken(..)
   , Dependency(..)
   , makeDependency
+  , makeUToken
   , formatDependencyTrees
   ) where
 
@@ -21,13 +22,11 @@ data Action = Function Function
             | FunctionIO FunctionIO
             | Inject Value
             | FilterComp
-            | GetListElem
-            | SetListElem
             | UntupleFst
             | UntupleSnd
             | UnzipFst
             | UnzipSnd
-            | MapComp [Dependency]
+            | MapComp [Dependency] UToken UToken
   deriving (Show, Lift)
 
 data UToken = UToken Int
@@ -83,8 +82,12 @@ formatDependencyTrees = T.concat . (formatDependencyTrees' "")
             [ "--" ]
 
         formatAction indentation = \case
-          MapComp subDeps ->
-            concat [ [ "MapComp:\n" ]
+          MapComp subDeps innerInput innerOutput ->
+            concat [ [ "MapComp(" ]
+                   , formatUToken innerInput
+                   , [ " -> " ]
+                   , formatUToken innerOutput
+                   , [ "):\n" ]
                    , formatDependencyTrees' (T.append indentation "|  ") subDeps
                    ]
           action ->