Displaying references in counterexamples.

In why3, references are internally stored in the record ref with one
field content (containing the value that is referenced). Disable
displaying the name of the field content of record ref in counterexamples.

modules/ref.mlw

@@ -13,7 +13,7 @@
 
 module Ref
 
-  type ref 'a = { mutable contents : 'a }
+  type ref 'a = { mutable contents "model_trace:" : 'a }
 
   function (!) (x: ref 'a) : 'a = x.contents