src/HOL/Tools/Predicate_Compile/code_prolog.ML

       case tss of
         [ts] => SOME (map (restore_term ctxt' constant_table) (ts ~~ map snd vs'))
       | _ => NONE
   in
     Quickcheck.Result
-      {counterexample = Option.map ((curry (op ~~)) (Term.add_free_names t [])) counterexample,
+      {counterexample = Option.map (pair true o curry (op ~~) (Term.add_free_names t [])) counterexample,
        evaluation_terms = Option.map (K []) counterexample, timings = [], reports = []}
   end;
 
 fun test_goals ctxt _ insts goals =
   let