diff --git a/app/Heap.hs b/app/Heap.hs
new file mode 100644
index 0000000..0dc79e6
--- /dev/null
+++ b/app/Heap.hs
@@ -0,0 +1,78 @@
+module Heap where
+
+import Code
+import CodeLens
+import Data.Foldable (traverse_)
+import qualified Data.Map as M
+import IR (Id(..))
+import Lens.Family2.State
+
+data Dereferenced
+  = FreeRef Int
+  | BoundRef Int Datum
+  | NoRef
+  deriving (Show, Eq)
+
+-- TRICKY: HeapRefs alone must not form a cycle other than the FreeRef case.
+deref' :: Heap -> Int -> Dereferenced
+deref' h@(Heap _ hmap) x =
+  case hmap M.!? x of
+    Just (HeapRef x') ->
+      if x == x'
+        then FreeRef x'
+        else deref' h x'
+    Just x' -> BoundRef x x'
+    _ -> NoRef
+
+derefHeap = deref' --TODO remove
+
+deref :: Int -> PrlgEnv Dereferenced
+deref = uses (cur . heap) . flip deref'
+
+writeHeap :: Int -> Datum -> PrlgEnv ()
+writeHeap a v =
+  cur . heap %= (\(Heap nxt m) -> Heap nxt $ M.insert a v m)
+
+allocHeap :: Int -> PrlgEnv Int
+allocHeap n = do
+  Heap nxt m <- use (cur . heap)
+  cur . heap .= Heap (nxt + n) m
+  pure nxt
+
+makeVar a = writeHeap a (HeapRef a)
+
+newHeapVar = head <$> newHeapVars 1
+
+newHeapVars n = do
+  base <- allocHeap n
+  let addrs = [base .. base + n - 1]
+  traverse_ makeVar addrs
+  pure addrs
+
+putHeapStruct addr s@(Struct Id {arity = arity}) = do
+  base <- allocHeap (arity + 1)
+  let paddrs = map (base +) [1 .. arity]
+  traverse_ makeVar paddrs
+  writeHeap base s
+  writeHeap addr (HeapRef base)
+  return $ map HeapRef paddrs
+
+heapStruct ::
+     Monad m
+  => (Datum -> m a)
+  -> (Datum -> [a] -> m a)
+  -> (Datum -> Int -> m a)
+  -> Heap
+  -> Int
+  -> m a
+heapStruct atom struct rec (Heap _ heap) hr = go [hr] hr
+  where
+    go visited ref
+      | rr@(HeapRef r) <- heap M.! ref =
+        if r == ref || r `elem` visited
+          then rec rr ref
+          else go (r : visited) r
+      | s@(Struct (Id _ arity)) <- heap M.! ref =
+        sequence [go (ref + i : visited) (ref + i) | i <- [1 .. arity]] >>=
+        struct s
+      | x <- heap M.! ref = atom x
diff --git a/app/Interpreter.hs b/app/Interpreter.hs
index 31c4c12..8531a27 100644
--- a/app/Interpreter.hs
+++ b/app/Interpreter.hs
@@ -50,6 +50,15 @@ proveStep :: InterpFn
 proveStep = do
   u <- use (cur . unis)
   h <- use (cur . hed)
+  {- tracing:
+  import Control.Monad.Trans.Class (lift)
+  import System.Console.Haskeline
+  g <- use (cur . gol)
+  lift $ do
+    outputStrLn $ "STEP (unis="++show u++")"
+    outputStrLn $ "head = "++ show h
+    outputStrLn $ "goal = "++ show g
+  -}
   case (u, h) of
     (0, []) -> goalStep
     (0, _) -> headStep h