diff --git a/extlib/benz/codegen.c b/extlib/benz/codegen.c
index a14a2886..d4f2c187 100644
--- a/extlib/benz/codegen.c
+++ b/extlib/benz/codegen.c
@@ -436,7 +436,7 @@ define_var(pic_state *pic, analyze_scope *scope, pic_sym *sym)
   int ret;
 
   if (search_scope(scope, sym)) {
-    if (scope->depth > 0 || (pic_dict_has(pic, pic->globals, sym) && ! pic_invalid_p(pic_cdr(pic, pic_dict_ref(pic, pic->globals, sym))))) {
+    if (scope->depth > 0 || (pic_dict_has(pic, pic->globals, sym) && ! pic_invalid_p(pic_box_ptr(pic_dict_ref(pic, pic->globals, sym))->value))) {
       pic_warnf(pic, "redefining variable: ~s", pic_obj_value(sym));
     }
     return;
@@ -782,15 +782,15 @@ index_local(codegen_context *cxt, pic_sym *sym)
 static int
 index_global(pic_state *pic, codegen_context *cxt, pic_sym *name)
 {
-  extern pic_value pic_vm_gref_slot(pic_state *, pic_sym *);
+  extern struct pic_box *pic_vm_gref_slot(pic_state *, pic_sym *);
   int pidx;
-  pic_value slot;
+  struct pic_box *slot;
 
   slot = pic_vm_gref_slot(pic, name);
 
   check_pool_size(pic, cxt);
   pidx = (int)cxt->plen++;
-  cxt->pool[pidx] = slot;
+  cxt->pool[pidx] = pic_obj_value(slot);
 
   return pidx;
 }
diff --git a/extlib/benz/gc.c b/extlib/benz/gc.c
index 2e7ae20f..a7096cca 100644
--- a/extlib/benz/gc.c
+++ b/extlib/benz/gc.c
@@ -35,6 +35,7 @@ union object {
   struct pic_port port;
   struct pic_error err;
   struct pic_lib lib;
+  struct pic_box box;
   struct pic_checkpoint cp;
 };
 
@@ -405,6 +406,12 @@ gc_mark_object(pic_state *pic, union object *obj)
     pic->heap->regs = reg;
     break;
   }
+  case PIC_TT_BOX: {
+    if (pic_obj_p(obj->box.value)) {
+      LOOP(pic_obj_ptr(obj->box.value));
+    }
+    break;
+  }
   case PIC_TT_CP: {
     if (obj->cp.prev) {
       gc_mark_object(pic, (union object *)obj->cp.prev);
@@ -597,6 +604,7 @@ gc_finalize_object(pic_state *pic, union object *obj)
   case PIC_TT_LIB:
   case PIC_TT_RECORD:
   case PIC_TT_CP:
+  case PIC_TT_BOX:
     break;
 
   case PIC_TT_NIL:
diff --git a/extlib/benz/include/picrin.h b/extlib/benz/include/picrin.h
index 30ec8a58..50737ea3 100644
--- a/extlib/benz/include/picrin.h
+++ b/extlib/benz/include/picrin.h
@@ -102,14 +102,15 @@ struct pic_state {
 
   pic_sym *uDEFINE, *uLAMBDA, *uIF, *uBEGIN, *uQUOTE, *uSETBANG, *uDEFINE_MACRO;
   pic_sym *uDEFINE_LIBRARY, *uIMPORT, *uEXPORT, *uCOND_EXPAND;
+
   pic_sym *uCONS, *uCAR, *uCDR, *uNILP, *uSYMBOLP, *uPAIRP;
   pic_sym *uADD, *uSUB, *uMUL, *uDIV, *uEQ, *uLT, *uLE, *uGT, *uGE, *uNOT;
 
   pic_value pCONS, pCAR, pCDR, pNILP, pPAIRP, pSYMBOLP, pNOT;
   pic_value pADD, pSUB, pMUL, pDIV, pEQ, pLT, pLE, pGT, pGE;
 
-  pic_value cCONS, cCAR, cCDR, cNILP, cPAIRP, cSYMBOLP, cNOT;
-  pic_value cADD, cSUB, cMUL, cDIV, cEQ, cLT, cLE, cGT, cGE;
+  struct pic_box *cCONS, *cCAR, *cCDR, *cNILP, *cPAIRP, *cSYMBOLP, *cNOT;
+  struct pic_box *cADD, *cSUB, *cMUL, *cDIV, *cEQ, *cLT, *cLE, *cGT, *cGE;
 
   struct pic_lib *PICRIN_BASE;
   struct pic_lib *PICRIN_USER;
@@ -268,6 +269,7 @@ pic_value pic_fdisplay(pic_state *, pic_value, xFILE *);
 #include "picrin/symbol.h"
 #include "picrin/vector.h"
 #include "picrin/reg.h"
+#include "picrin/box.h"
 
 #if defined(__cplusplus)
 }
diff --git a/extlib/benz/include/picrin/box.h b/extlib/benz/include/picrin/box.h
new file mode 100644
index 00000000..dcfa676a
--- /dev/null
+++ b/extlib/benz/include/picrin/box.h
@@ -0,0 +1,34 @@
+/**
+ * See Copyright Notice in picrin.h
+ */
+
+#ifndef PICRIN_BOX_H
+#define PICRIN_BOX_H
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+struct pic_box {
+  PIC_OBJECT_HEADER
+  pic_value value;
+};
+
+#define pic_box_p(v) (pic_type(v) == PIC_TT_BOX)
+#define pic_box_ptr(v) ((struct pic_box *)pic_ptr(v))
+
+PIC_INLINE struct pic_box *
+pic_box(pic_state *pic, pic_value value)
+{
+  struct pic_box *box;
+
+  box = (struct pic_box *)pic_obj_alloc(pic, sizeof(struct pic_box), PIC_TT_BOX);
+  box->value = value;
+  return box;
+}
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif
diff --git a/extlib/benz/include/picrin/type.h b/extlib/benz/include/picrin/type.h
index 0df475a8..4de8d279 100644
--- a/extlib/benz/include/picrin/type.h
+++ b/extlib/benz/include/picrin/type.h
@@ -156,6 +156,7 @@ enum pic_tt {
   PIC_TT_DICT,
   PIC_TT_REG,
   PIC_TT_RECORD,
+  PIC_TT_BOX,
   PIC_TT_CXT,
   PIC_TT_IREP,
   PIC_TT_CP
@@ -313,6 +314,8 @@ pic_type_repr(enum pic_tt tt)
     return "dict";
   case PIC_TT_REG:
     return "reg";
+  case PIC_TT_BOX:
+    return "box";
   case PIC_TT_RECORD:
     return "record";
   case PIC_TT_CP:
diff --git a/extlib/benz/state.c b/extlib/benz/state.c
index 99127266..86f3d88a 100644
--- a/extlib/benz/state.c
+++ b/extlib/benz/state.c
@@ -127,7 +127,7 @@ static void
 pic_init_core(pic_state *pic)
 {
   void pic_define_syntactic_keyword_(pic_state *, struct pic_env *, pic_sym *, pic_sym *);
-  pic_value pic_vm_gref_slot(pic_state *, pic_sym *);
+  struct pic_box *pic_vm_gref_slot(pic_state *, pic_sym *);
 
   pic_init_features(pic);
 
@@ -434,22 +434,22 @@ pic_open(pic_allocf allocf, void *userdata)
   /* turn on GC */
   pic->gc_enable = true;
 
-  pic->cCONS = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cCAR = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cCDR = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cNILP = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cSYMBOLP = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cPAIRP = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cNOT = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cADD = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cSUB = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cMUL = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cDIV = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cEQ = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cLT = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cLE = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cGT = pic_cons(pic, pic_false_value(), pic_invalid_value());
-  pic->cGE = pic_cons(pic, pic_false_value(), pic_invalid_value());
+  pic->cCONS = pic_box(pic, pic_invalid_value());
+  pic->cCAR = pic_box(pic, pic_invalid_value());
+  pic->cCDR = pic_box(pic, pic_invalid_value());
+  pic->cNILP = pic_box(pic, pic_invalid_value());
+  pic->cSYMBOLP = pic_box(pic, pic_invalid_value());
+  pic->cPAIRP = pic_box(pic, pic_invalid_value());
+  pic->cNOT = pic_box(pic, pic_invalid_value());
+  pic->cADD = pic_box(pic, pic_invalid_value());
+  pic->cSUB = pic_box(pic, pic_invalid_value());
+  pic->cMUL = pic_box(pic, pic_invalid_value());
+  pic->cDIV = pic_box(pic, pic_invalid_value());
+  pic->cEQ = pic_box(pic, pic_invalid_value());
+  pic->cLT = pic_box(pic, pic_invalid_value());
+  pic->cLE = pic_box(pic, pic_invalid_value());
+  pic->cGT = pic_box(pic, pic_invalid_value());
+  pic->cGE = pic_box(pic, pic_invalid_value());
 
   pic_init_core(pic);
 
diff --git a/extlib/benz/vm.c b/extlib/benz/vm.c
index d586832b..2f291111 100644
--- a/extlib/benz/vm.c
+++ b/extlib/benz/vm.c
@@ -386,32 +386,35 @@ pic_get_args(pic_state *pic, const char *format, ...)
   return argc;
 }
 
-pic_value
+struct pic_box *
 pic_vm_gref_slot(pic_state *pic, pic_sym *uid)
 {
-  pic_value slot;
+  struct pic_box *box;
 
   if (pic_dict_has(pic, pic->globals, uid)) {
-    return pic_dict_ref(pic, pic->globals, uid);
+    return pic_box_ptr(pic_dict_ref(pic, pic->globals, uid));
   }
-  slot = pic_cons(pic, pic_obj_value(uid), pic_invalid_value());
-  pic_dict_set(pic, pic->globals, uid, slot);
-  return slot;
+  box = pic_box(pic, pic_invalid_value());
+  pic_dict_set(pic, pic->globals, uid, pic_obj_value(box));
+  return box;
 }
 
 static pic_value
-vm_gref(pic_state *pic, pic_value slot)
+vm_gref(pic_state *pic, struct pic_box *slot, pic_sym *uid)
 {
-  if (pic_invalid_p(pic_cdr(pic, slot))) {
-    pic_errorf(pic, "uninitialized global variable: ~a", pic_car(pic, slot));
+  if (pic_invalid_p(slot->value)) {
+    if (uid == NULL) {
+      uid = pic_intern(pic, "unknown"); /* FIXME */
+    }
+    pic_errorf(pic, "uninitialized global variable: ~a", uid);
   }
-  return pic_cdr(pic, slot);
+  return slot->value;
 }
 
 static void
-vm_gset(pic_state *pic, pic_value slot, pic_value value)
+vm_gset(struct pic_box *slot, pic_value value)
 {
-  pic_set_cdr(pic, slot, value);
+  slot->value = value;
 }
 
 static void
@@ -630,11 +633,11 @@ pic_apply(pic_state *pic, struct pic_proc *proc, pic_value args)
       NEXT;
     }
     CASE(OP_GREF) {
-      PUSH(vm_gref(pic, pic->ci->irep->pool[c.u.i]));
+      PUSH(vm_gref(pic, pic_box_ptr(pic->ci->irep->pool[c.u.i]), NULL)); /* FIXME */
       NEXT;
     }
     CASE(OP_GSET) {
-      vm_gset(pic, pic->ci->irep->pool[c.u.i], POP());
+      vm_gset(pic_box_ptr(pic->ci->irep->pool[c.u.i]), POP());
       PUSH(pic_undef_value());
       NEXT;
     }
@@ -847,7 +850,7 @@ pic_apply(pic_state *pic, struct pic_proc *proc, pic_value args)
     }
 
 #define check_condition(name, n) do {                                   \
-      if (! pic_eq_p(pic->p##name, pic_cdr(pic, pic->c##name)))         \
+      if (! pic_eq_p(pic->p##name, pic->c##name->value))                \
         goto L_CALL;                                                    \
       if (c.u.i != n + 1)                                               \
         goto L_CALL;                                                    \
@@ -1161,7 +1164,7 @@ pic_ref(pic_state *pic, struct pic_lib *lib, const char *name)
     pic_errorf(pic, "symbol \"%s\" not defined in library ~s", name, lib->name);
   }
 
-  return vm_gref(pic, pic_vm_gref_slot(pic, uid));
+  return vm_gref(pic, pic_vm_gref_slot(pic, uid), uid);
 }
 
 void
@@ -1175,7 +1178,7 @@ pic_set(pic_state *pic, struct pic_lib *lib, const char *name, pic_value val)
     pic_errorf(pic, "symbol \"%s\" not defined in library ~s", name, lib->name);
   }
 
-  vm_gset(pic, pic_vm_gref_slot(pic, uid), val);
+  vm_gset(pic_vm_gref_slot(pic, uid), val);
 }
 
 pic_value