linked_list_rev: use of ghost variables

examples/linked_list_rev.mlw

@@ -70,14 +70,14 @@ module InPlaceRev
 
   val next : ref (map loc loc)
 
-  let in_place_reverse (l:loc) (lM:list loc) : loc
+  let in_place_reverse (l:loc) (ghost lM:list loc) : loc
     requires { list_seg l !next lM null }
     ensures  { list_seg result !next (reverse lM) null }
   = 'Init:
     let p = ref l in
-    let pM = ref lM in
+    let ghost pM = ref lM in
     let r = ref null in
-    let rM = ref (Nil : list loc) in
+    let ghost rM = ref (Nil : list loc) in
     while !p <> null do
       invariant { list_seg !p !next !pM null }
       invariant { list_seg !r !next !rM null }