diff --git a/app/Interpreter.hs b/app/Interpreter.hs
index bb47144..774c13d 100644
--- a/app/Interpreter.hs
+++ b/app/Interpreter.hs
@@ -129,6 +129,16 @@ proveStep = St.get >>= go
         unify (HeapRef _) VoidRef = uok
         unify VoidRef (HeapRef _) = uok
         {- actual HeapRefs, these are dereferenced and then unified (sometimes with copying) -}
+        unify (HeapRef hr) (HeapRef gr)
+          | BoundRef ha _ <- deref hr
+          , BoundRef ga _ <- deref gr
+          , ha == ga = uok
+          | FreeRef ha <- deref hr
+          , BoundRef ga _ <- deref gr = setHeap ha (HeapRef ga)
+          | BoundRef ha _ <- deref hr
+          , FreeRef ga <- deref gr = setHeap ga (HeapRef ha)
+          | FreeRef ha <- deref hr
+          , FreeRef ga <- deref gr = setHeap ha (HeapRef ga)
         unify (HeapRef hr') g =
           case deref hr' of
             FreeRef hr ->