Prevent the compiler from statically allocating an persistent variant (that is...

Prevent the compiler from statically allocating an persistent variant (that is mutated by the OCaml runtime)

Conflicts:
	runtime/value.ml

runtime/value.ml

@@ -212,7 +212,9 @@ let rec flatten = function
   | q -> q
 
 let eval_lazy_concat v =
-  let accu = Obj.magic (Pair (nil,Absent,Mono)) in
+  (* prevent the ocamlopt from statically allocating the
+     value Pair(nil, Absent) *)
+  let accu = Obj.(magic (dup (repr (Pair (nil, Absent))))) in
   let rec aux accu = function
     | Concat (x,y, _) -> aux (append_cdr accu x) y
     | v -> set_cdr accu v