diff --git a/extlib/benz/bool.c b/extlib/benz/bool.c
index 15d857e6..f01e65ac 100644
--- a/extlib/benz/bool.c
+++ b/extlib/benz/bool.c
@@ -100,8 +100,8 @@ internal_equal_p(pic_state *pic, pic_value x, pic_value y, int depth, khash_t(m)
     id1 = pic_id_ptr(pic, x);
     id2 = pic_id_ptr(pic, y);
 
-    s1 = pic_find_identifier(pic, pic_obj_value(id1->u.id), id1->env);
-    s2 = pic_find_identifier(pic, pic_obj_value(id2->u.id), id2->env);
+    s1 = pic_find_identifier(pic, pic_obj_value(id1->u.id), pic_obj_value(id1->env));
+    s2 = pic_find_identifier(pic, pic_obj_value(id2->u.id), pic_obj_value(id2->env));
 
     return pic_eq_p(pic, s1, s2);
   }
diff --git a/extlib/benz/eval.c b/extlib/benz/eval.c
index d8b86146..9c2638a1 100644
--- a/extlib/benz/eval.c
+++ b/extlib/benz/eval.c
@@ -876,8 +876,7 @@ pic_value
 pic_eval(pic_state *pic, pic_value program, const char *lib)
 {
   const char *prev_lib = pic_current_library(pic);
-  struct pic_env *env;
-  pic_value r;
+  pic_value env, r;
 
   env = pic_library_environment(pic, lib);
 
diff --git a/extlib/benz/include/picrin.h b/extlib/benz/include/picrin.h
index d650f8db..d0e75128 100644
--- a/extlib/benz/include/picrin.h
+++ b/extlib/benz/include/picrin.h
@@ -51,10 +51,8 @@ typedef struct {
 } pic_value;
 #endif
 
-struct pic_object;
 struct pic_port;
 struct pic_error;
-struct pic_env;
 
 typedef void *(*pic_allocf)(void *userdata, void *ptr, size_t n);
 
@@ -291,9 +289,9 @@ void pic_close_port(pic_state *, struct pic_port *port);
 pic_value pic_read(pic_state *, struct pic_port *);
 pic_value pic_read_cstr(pic_state *, const char *);
 
-pic_value pic_expand(pic_state *, pic_value, struct pic_env *);
+pic_value pic_expand(pic_state *, pic_value program, pic_value env);
 
-pic_value pic_eval(pic_state *, pic_value, const char *);
+pic_value pic_eval(pic_state *, pic_value program, const char *lib);
 
 void pic_load(pic_state *, struct pic_port *);
 void pic_load_cstr(pic_state *, const char *);
@@ -360,7 +358,7 @@ void pic_fprintf(pic_state *, struct pic_port *, const char *, ...);
 pic_value pic_display(pic_state *, pic_value);
 pic_value pic_fdisplay(pic_state *, pic_value, xFILE *);
 
-struct pic_env *pic_library_environment(pic_state *, const char *);
+pic_value pic_library_environment(pic_state *, const char *);
 
 #if DEBUG
 # define pic_debug(pic,obj) pic_fwrite(pic,obj,xstderr)
diff --git a/extlib/benz/include/picrin/object.h b/extlib/benz/include/picrin/object.h
index b9861401..38a155bc 100644
--- a/extlib/benz/include/picrin/object.h
+++ b/extlib/benz/include/picrin/object.h
@@ -17,6 +17,8 @@ KHASH_DECLARE(weak, struct pic_object *, pic_value)
   unsigned char tt;                             \
   char gc_mark;
 
+struct pic_object;              /* defined in gc.c */
+
 struct pic_basic {
   PIC_OBJECT_HEADER
 };
@@ -141,11 +143,11 @@ struct pic_checkpoint {
 #define pic_weak_ptr(pic, o) ((struct pic_weak *)pic_obj_ptr(o))
 #define pic_data_ptr(pic, o) ((struct pic_data *)pic_obj_ptr(o))
 #define pic_proc_ptr(pic, o) ((struct pic_proc *)pic_obj_ptr(o))
+#define pic_env_ptr(pic, o) ((struct pic_env *)pic_obj_ptr(o))
 #define pic_context_ptr(o) ((struct pic_context *)pic_obj_ptr(o))
 #define pic_rec_ptr(v) ((struct pic_record *)pic_obj_ptr(v))
 #define pic_error_ptr(v) ((struct pic_error *)pic_obj_ptr(v))
 #define pic_port_ptr(v) ((struct pic_port *)pic_obj_ptr(v))
-#define pic_env_ptr(v) ((struct pic_env *)pic_obj_ptr(v))
 
 #define pic_obj_p(pic,v) (pic_vtype(pic,v) == PIC_IVAL_END)
 #define pic_env_p(pic, v) (pic_type(pic, v) == PIC_TYPE_ENV)
@@ -168,16 +170,16 @@ struct pic_object *pic_obj_alloc(pic_state *, size_t, int type);
     if (tolen - at < e - s) pic_errorf(pic, "invalid range");   \
   } while (0)
 
-pic_value pic_make_identifier(pic_state *, pic_value id, struct pic_env *);
+pic_value pic_make_identifier(pic_state *, pic_value id, pic_value env);
 pic_value pic_make_proc(pic_state *, pic_func_t, int, pic_value *);
 pic_value pic_make_proc_irep(pic_state *, struct pic_irep *, struct pic_context *);
 struct pic_record *pic_make_rec(pic_state *, pic_value, pic_value);
 struct pic_error *pic_make_error(pic_state *, const char *, const char *, pic_value);
-struct pic_env *pic_make_env(pic_state *, struct pic_env *);
+pic_value pic_make_env(pic_state *, pic_value env);
 
-pic_value pic_add_identifier(pic_state *, pic_value id, struct pic_env *);
-pic_value pic_put_identifier(pic_state *, pic_value id, pic_value uid, struct pic_env *);
-pic_value pic_find_identifier(pic_state *, pic_value id, struct pic_env *);
+pic_value pic_add_identifier(pic_state *, pic_value id, pic_value env);
+pic_value pic_put_identifier(pic_state *, pic_value id, pic_value uid, pic_value env);
+pic_value pic_find_identifier(pic_state *, pic_value id, pic_value env);
 pic_value pic_id_name(pic_state *, pic_value id);
 
 void pic_rope_incref(pic_state *, struct pic_rope *);
diff --git a/extlib/benz/lib.c b/extlib/benz/lib.c
index 2a5d7d34..a7e9336a 100644
--- a/extlib/benz/lib.c
+++ b/extlib/benz/lib.c
@@ -31,23 +31,26 @@ get_library(pic_state *pic, const char *lib)
   return libp;
 }
 
-static struct pic_env *
+static pic_value
 make_library_env(pic_state *pic, pic_value name)
 {
   struct pic_env *env;
+  pic_value e;
 
   env = (struct pic_env *)pic_obj_alloc(pic, sizeof(struct pic_env), PIC_TYPE_ENV);
   env->up = NULL;
   env->lib = pic_str_ptr(pic, name);
   kh_init(env, &env->map);
 
-  /* set up default environment */
-  pic_put_identifier(pic, pic->sDEFINE_LIBRARY, pic->sDEFINE_LIBRARY, env);
-  pic_put_identifier(pic, pic->sIMPORT, pic->sIMPORT, env);
-  pic_put_identifier(pic, pic->sEXPORT, pic->sEXPORT, env);
-  pic_put_identifier(pic, pic->sCOND_EXPAND, pic->sCOND_EXPAND, env);
+  e = pic_obj_value(env);
 
-  return env;
+  /* set up default environment */
+  pic_put_identifier(pic, pic->sDEFINE_LIBRARY, pic->sDEFINE_LIBRARY, e);
+  pic_put_identifier(pic, pic->sIMPORT, pic->sIMPORT, e);
+  pic_put_identifier(pic, pic->sEXPORT, pic->sEXPORT, e);
+  pic_put_identifier(pic, pic->sCOND_EXPAND, pic->sCOND_EXPAND, e);
+
+  return e;
 }
 
 void
@@ -55,8 +58,7 @@ pic_make_library(pic_state *pic, const char *lib)
 {
   khash_t(ltable) *h = &pic->ltable;
   const char *old_lib;
-  struct pic_env *env;
-  pic_value name, exports;
+  pic_value name, env, exports;
   khiter_t it;
   int ret;
 
@@ -74,7 +76,7 @@ pic_make_library(pic_state *pic, const char *lib)
   }
 
   kh_val(h, it).name = pic_str_ptr(pic, name);
-  kh_val(h, it).env = env;
+  kh_val(h, it).env = pic_env_ptr(pic, env);
   kh_val(h, it).exports = pic_dict_ptr(pic, exports);
 
   if (pic->lib) {
@@ -100,10 +102,10 @@ pic_current_library(pic_state *pic)
   return pic_str(pic, pic_obj_value(pic->lib->name));
 }
 
-struct pic_env *
+pic_value
 pic_library_environment(pic_state *pic, const char *lib)
 {
-  return get_library(pic, lib)->env;
+  return pic_obj_value(get_library(pic, lib)->env);
 }
 
 void
@@ -116,11 +118,11 @@ pic_import(pic_state *pic, const char *lib)
   libp = get_library(pic, lib);
 
   while (pic_dict_next(pic, pic_obj_value(libp->exports), &it, &name, &realname)) {
-    uid = pic_find_identifier(pic, realname, libp->env);
+    uid = pic_find_identifier(pic, realname, pic_obj_value(libp->env));
     if (! pic_weak_has(pic, pic->globals, uid) && ! pic_weak_has(pic, pic->macros, uid)) {
       pic_errorf(pic, "attempted to export undefined variable '~s'", realname);
     }
-    pic_put_identifier(pic, name, uid, pic->lib->env);
+    pic_put_identifier(pic, name, uid, pic_obj_value(pic->lib->env));
   }
 }
 
@@ -192,12 +194,12 @@ pic_lib_library_import(pic_state *pic)
     realname = pic_dict_ref(pic, pic_obj_value(libp->exports), name);
   }
 
-  uid = pic_find_identifier(pic, realname, libp->env);
+  uid = pic_find_identifier(pic, realname, pic_obj_value(libp->env));
   if (! pic_weak_has(pic, pic->globals, uid) && ! pic_weak_has(pic, pic->macros, uid)) {
     pic_errorf(pic, "attempted to export undefined variable '~s'", realname);
   }
 
-  pic_put_identifier(pic, alias, uid, pic->lib->env);
+  pic_put_identifier(pic, alias, uid, pic_obj_value(pic->lib->env));
 
   return pic_undef_value(pic);
 }
diff --git a/extlib/benz/macro.c b/extlib/benz/macro.c
index c9f69ec0..99ca4de8 100644
--- a/extlib/benz/macro.c
+++ b/extlib/benz/macro.c
@@ -7,30 +7,29 @@
 
 KHASH_DEFINE(env, pic_id *, pic_sym *, kh_ptr_hash_func, kh_ptr_hash_equal)
 
-struct pic_env *
-pic_make_env(pic_state *pic, struct pic_env *up)
+pic_value
+pic_make_env(pic_state *pic, pic_value up)
 {
   struct pic_env *env;
 
-  assert(up != NULL);
-
   env = (struct pic_env *)pic_obj_alloc(pic, sizeof(struct pic_env), PIC_TYPE_ENV);
-  env->up = up;
+  env->up = pic_env_ptr(pic, up);
   env->lib = NULL;
   kh_init(env, &env->map);
-  return env;
+
+  return pic_obj_value(env);
 }
 
 pic_value
-pic_add_identifier(pic_state *pic, pic_value id, struct pic_env *env)
+pic_add_identifier(pic_state *pic, pic_value id, pic_value env)
 {
   const char *name;
   pic_value uid, str;
 
   name = pic_str(pic, pic_id_name(pic, id));
 
-  if (env->up == NULL && pic_sym_p(pic, id)) { /* toplevel & public */
-    str = pic_strf_value(pic, "%s/%s", pic_str(pic, pic_obj_value(env->lib)), name);
+  if (pic_env_ptr(pic, env)->up == NULL && pic_sym_p(pic, id)) { /* toplevel & public */
+    str = pic_strf_value(pic, "~a/%s", pic_obj_value(pic_env_ptr(pic, env)->lib), name);
   } else {
     str = pic_strf_value(pic, ".%s.%d", name, pic->ucnt++);
   }
@@ -40,55 +39,63 @@ pic_add_identifier(pic_state *pic, pic_value id, struct pic_env *env)
 }
 
 pic_value
-pic_put_identifier(pic_state *pic, pic_value id, pic_value uid, struct pic_env *env)
+pic_put_identifier(pic_state *pic, pic_value id, pic_value uid, pic_value env)
 {
   khiter_t it;
   int ret;
 
-  it = kh_put(env, &env->map, pic_id_ptr(pic, id), &ret);
-  kh_val(&env->map, it) = pic_sym_ptr(pic, uid);
+  it = kh_put(env, &pic_env_ptr(pic, env)->map, pic_id_ptr(pic, id), &ret);
+  kh_val(&pic_env_ptr(pic, env)->map, it) = pic_sym_ptr(pic, uid);
 
   return uid;
 }
 
 static bool
-search_scope(pic_state *pic, pic_value id, struct pic_env *env, pic_value *uid)
+search_scope(pic_state *pic, pic_value id, pic_value env, pic_value *uid)
 {
   khiter_t it;
 
-  it = kh_get(env, &env->map, pic_id_ptr(pic, id));
-  if (it == kh_end(&env->map)) {
+  it = kh_get(env, &pic_env_ptr(pic, env)->map, pic_id_ptr(pic, id));
+  if (it == kh_end(&pic_env_ptr(pic, env)->map)) {
     return false;
   }
-  *uid = pic_obj_value(kh_val(&env->map, it));
+  *uid = pic_obj_value(kh_val(&pic_env_ptr(pic, env)->map, it));
   return true;
 }
 
 static bool
-search(pic_state *pic, pic_value id, struct pic_env *env, pic_value *uid)
+search(pic_state *pic, pic_value id, pic_value env, pic_value *uid)
 {
-  while (env != NULL) {
-    if (search_scope(pic, id, env, uid)) {
+  struct pic_env *e;
+
+  while (1) {
+    if (search_scope(pic, id, env, uid))
       return true;
-    }
-    env = env->up;
+    e = pic_env_ptr(pic, env)->up;
+    if (e == NULL)
+      break;
+    env = pic_obj_value(e);
   }
   return false;
 }
 
 pic_value
-pic_find_identifier(pic_state *pic, pic_value id, struct pic_env *env)
+pic_find_identifier(pic_state *pic, pic_value id, pic_value env)
 {
+  struct pic_env *e;
   pic_value uid;
 
   while (! search(pic, id, env, &uid)) {
     if (pic_sym_p(pic, id)) {
-      while (env->up != NULL) {
-        env = env->up;
+      while (1) {
+        e = pic_env_ptr(pic, env);
+        if (e->up == NULL)
+          break;
+        env = pic_obj_value(e->up);
       }
       return pic_add_identifier(pic, id, env);
     }
-    env = pic_id_ptr(pic, id)->env;              /* do not overwrite id first */
+    env = pic_obj_value(pic_id_ptr(pic, id)->env); /* do not overwrite id first */
     id = pic_obj_value(pic_id_ptr(pic, id)->u.id);
   }
   return uid;
@@ -127,18 +134,18 @@ shadow_macro(pic_state *pic, pic_value uid)
   }
 }
 
-static pic_value expand(pic_state *, pic_value, struct pic_env *, pic_value);
-static pic_value expand_lambda(pic_state *, pic_value, struct pic_env *);
+static pic_value expand(pic_state *, pic_value expr, pic_value env, pic_value deferred);
+static pic_value expand_lambda(pic_state *, pic_value expr, pic_value env);
 
 static pic_value
-expand_var(pic_state *pic, pic_value id, struct pic_env *env, pic_value deferred)
+expand_var(pic_state *pic, pic_value id, pic_value env, pic_value deferred)
 {
   pic_value mac, functor;
 
   functor = pic_find_identifier(pic, id, env);
 
   if (find_macro(pic, functor, &mac)) {
-    return expand(pic, pic_call(pic, mac, 2, id, pic_obj_value(env)), env, deferred);
+    return expand(pic, pic_call(pic, mac, 2, id, env), env, deferred);
   }
   return functor;
 }
@@ -150,7 +157,7 @@ expand_quote(pic_state *pic, pic_value expr)
 }
 
 static pic_value
-expand_list(pic_state *pic, pic_value obj, struct pic_env *env, pic_value deferred)
+expand_list(pic_state *pic, pic_value obj, pic_value env, pic_value deferred)
 {
   size_t ai = pic_enter(pic);
   pic_value x, head, tail;
@@ -179,7 +186,7 @@ expand_defer(pic_state *pic, pic_value expr, pic_value deferred)
 }
 
 static void
-expand_deferred(pic_state *pic, pic_value deferred, struct pic_env *env)
+expand_deferred(pic_state *pic, pic_value deferred, pic_value env)
 {
   pic_value defer, val, src, dst, it;
 
@@ -198,10 +205,10 @@ expand_deferred(pic_state *pic, pic_value deferred, struct pic_env *env)
 }
 
 static pic_value
-expand_lambda(pic_state *pic, pic_value expr, struct pic_env *env)
+expand_lambda(pic_state *pic, pic_value expr, pic_value env)
 {
   pic_value formal, body;
-  struct pic_env *in;
+  pic_value in;
   pic_value a, deferred;
 
   in = pic_make_env(pic, env);
@@ -224,7 +231,7 @@ expand_lambda(pic_state *pic, pic_value expr, struct pic_env *env)
 }
 
 static pic_value
-expand_define(pic_state *pic, pic_value expr, struct pic_env *env, pic_value deferred)
+expand_define(pic_state *pic, pic_value expr, pic_value env, pic_value deferred)
 {
   pic_value id, uid, val;
 
@@ -240,7 +247,7 @@ expand_define(pic_state *pic, pic_value expr, struct pic_env *env, pic_value def
 }
 
 static pic_value
-expand_defmacro(pic_state *pic, pic_value expr, struct pic_env *env)
+expand_defmacro(pic_state *pic, pic_value expr, pic_value env)
 {
   pic_value pic_compile(pic_state *, pic_value);
   pic_value id, uid, val;
@@ -261,7 +268,7 @@ expand_defmacro(pic_state *pic, pic_value expr, struct pic_env *env)
 }
 
 static pic_value
-expand_node(pic_state *pic, pic_value expr, struct pic_env *env, pic_value deferred)
+expand_node(pic_state *pic, pic_value expr, pic_value env, pic_value deferred)
 {
   switch (pic_type(pic, expr)) {
   case PIC_TYPE_ID:
@@ -294,7 +301,7 @@ expand_node(pic_state *pic, pic_value expr, struct pic_env *env, pic_value defer
       }
 
       if (find_macro(pic, functor, &mac)) {
-        return expand(pic, pic_call(pic, mac, 2, expr, pic_obj_value(env)), env, deferred);
+        return expand(pic, pic_call(pic, mac, 2, expr, env), env, deferred);
       }
     }
     return expand_list(pic, expr, env, deferred);
@@ -305,7 +312,7 @@ expand_node(pic_state *pic, pic_value expr, struct pic_env *env, pic_value defer
 }
 
 static pic_value
-expand(pic_state *pic, pic_value expr, struct pic_env *env, pic_value deferred)
+expand(pic_state *pic, pic_value expr, pic_value env, pic_value deferred)
 {
   size_t ai = pic_enter(pic);
   pic_value v;
@@ -318,7 +325,7 @@ expand(pic_state *pic, pic_value expr, struct pic_env *env, pic_value deferred)
 }
 
 pic_value
-pic_expand(pic_state *pic, pic_value expr, struct pic_env *env)
+pic_expand(pic_state *pic, pic_value expr, pic_value env)
 {
   pic_value v, deferred;
 
diff --git a/extlib/benz/proc.c b/extlib/benz/proc.c
index f2b758d2..0a1f20db 100644
--- a/extlib/benz/proc.c
+++ b/extlib/benz/proc.c
@@ -890,8 +890,7 @@ pic_defvar(pic_state *pic, const char *name, pic_value init, pic_value conv)
 void
 pic_define(pic_state *pic, const char *lib, const char *name, pic_value val)
 {
-  pic_value sym, uid;
-  struct pic_env *env;
+  pic_value sym, uid, env;
 
   sym = pic_intern_cstr(pic, name);
 
@@ -907,8 +906,7 @@ pic_define(pic_state *pic, const char *lib, const char *name, pic_value val)
 pic_value
 pic_ref(pic_state *pic, const char *lib, const char *name)
 {
-  pic_value sym, uid;
-  struct pic_env *env;
+  pic_value sym, uid, env;
 
   sym = pic_intern_cstr(pic, name);
 
@@ -925,8 +923,7 @@ pic_ref(pic_state *pic, const char *lib, const char *name)
 void
 pic_set(pic_state *pic, const char *lib, const char *name, pic_value val)
 {
-  pic_value sym, uid;
-  struct pic_env *env;
+  pic_value sym, uid, env;
 
   sym = pic_intern_cstr(pic, name);
 
diff --git a/extlib/benz/state.c b/extlib/benz/state.c
index 987d80ab..065ad248 100644
--- a/extlib/benz/state.c
+++ b/extlib/benz/state.c
@@ -77,17 +77,17 @@ pic_features(pic_state *pic)
   return pic->features;
 }
 
-#define import_builtin_syntax(name) do {                \
-    pic_value nick, real;                               \
-    nick = pic_intern_lit(pic, "builtin:" name);        \
-    real = pic_intern_lit(pic, name);                   \
-    pic_put_identifier(pic, nick, real, pic->lib->env); \
+#define import_builtin_syntax(name) do {                                \
+    pic_value nick, real;                                               \
+    nick = pic_intern_lit(pic, "builtin:" name);                        \
+    real = pic_intern_lit(pic, name);                                   \
+    pic_put_identifier(pic, nick, real, pic_obj_value(pic->lib->env));  \
   } while (0)
 
-#define declare_vm_procedure(name) do {                 \
-    pic_value sym;                                      \
-    sym = pic_intern_lit(pic, name);                    \
-    pic_put_identifier(pic, sym, sym, pic->lib->env);   \
+#define declare_vm_procedure(name) do {                                 \
+    pic_value sym;                                                      \
+    sym = pic_intern_lit(pic, name);                                    \
+    pic_put_identifier(pic, sym, sym, pic_obj_value(pic->lib->env));    \
   } while (0)
 
 void pic_init_bool(pic_state *);
diff --git a/extlib/benz/symbol.c b/extlib/benz/symbol.c
index f8dabf80..20941f70 100644
--- a/extlib/benz/symbol.c
+++ b/extlib/benz/symbol.c
@@ -35,13 +35,13 @@ pic_intern(pic_state *pic, pic_value str)
 }
 
 pic_value
-pic_make_identifier(pic_state *pic, pic_value base, struct pic_env *env)
+pic_make_identifier(pic_state *pic, pic_value base, pic_value env)
 {
   pic_id *id;
 
   id = (pic_id *)pic_obj_alloc(pic, sizeof(pic_id), PIC_TYPE_ID);
   id->u.id = pic_id_ptr(pic, base);
-  id->env = env;
+  id->env = pic_env_ptr(pic, env);
 
   return pic_obj_value(id);
 }
@@ -131,7 +131,7 @@ pic_symbol_make_identifier(pic_state *pic)
   pic_assert_type(pic, id, id);
   pic_assert_type(pic, env, env);
 
-  return pic_make_identifier(pic, id, pic_env_ptr(env));
+  return pic_make_identifier(pic, id, env);
 }
 
 static pic_value