diff --git a/code/imperative-programming/value_restriction-13.rawscript b/code/imperative-programming/value_restriction-13.rawscript
index c3965b3..e40eb01 100644
--- a/code/imperative-programming/value_restriction-13.rawscript
+++ b/code/imperative-programming/value_restriction-13.rawscript
@@ -1,5 +1,5 @@
 # module Concat_list : sig
-    type 'a t
+    type +'a t
     val empty : 'a t
     val singleton : 'a -> 'a t
     val concat  : 'a t -> 'a t -> 'a t  (* constant time *)