diff --git a/Makefile b/Makefile
index 757c480e..6aab7070 100644
--- a/Makefile
+++ b/Makefile
@@ -49,7 +49,7 @@ src/init_contrib.c:
 lib/libbenz.a: $(BENZ_OBJS)
 	$(AR) $(ARFLAGS) $@ $(BENZ_OBJS)
 
-%.o: extlib/benz/include/picrin.h extlib/benz/include/picrin/*.h
+$(PICRIN_OBJS) $(CONTRIB_OBJS): extlib/benz/include/picrin.h extlib/benz/include/picrin/*.h
 
 doc: docs/*.rst docs/contrib.rst
 	$(MAKE) -C docs html
diff --git a/extlib/benz/include/picrin.h b/extlib/benz/include/picrin.h
index 84ab6f45..46da1afe 100644
--- a/extlib/benz/include/picrin.h
+++ b/extlib/benz/include/picrin.h
@@ -203,13 +203,16 @@ struct pic_proc *pic_compile(pic_state *, pic_value, struct pic_lib *);
 pic_value pic_macroexpand(pic_state *, pic_value, struct pic_lib *);
 
 void pic_in_library(pic_state *, pic_value);
-struct pic_lib *pic_open_library(pic_state *, pic_value);
+struct pic_lib *pic_make_library(pic_state *, pic_value);
 struct pic_lib *pic_find_library(pic_state *, pic_value);
 
 #define pic_deflibrary(pic, spec)                                       \
   for (((assert(pic->prev_lib == NULL)),                                \
         (pic->prev_lib = pic->lib),                                     \
-        (pic->lib = pic_open_library(pic, pic_read_cstr(pic, (spec))))); \
+        (pic->lib = pic_find_library(pic, pic_read_cstr(pic, (spec)))), \
+        (pic->lib = pic->lib                                            \
+         ? pic->lib                                                     \
+         : pic_make_library(pic, pic_read_cstr(pic, (spec)))));         \
        pic->prev_lib != NULL;                                           \
        ((pic->lib = pic->prev_lib),                                     \
         (pic->prev_lib = NULL)))
diff --git a/extlib/benz/lib.c b/extlib/benz/lib.c
index ffa0583b..8eff0bb7 100644
--- a/extlib/benz/lib.c
+++ b/extlib/benz/lib.c
@@ -5,21 +5,14 @@
 #include "picrin.h"
 
 struct pic_lib *
-pic_open_library(pic_state *pic, pic_value name)
+pic_make_library(pic_state *pic, pic_value name)
 {
   struct pic_lib *lib;
   struct pic_env *env;
   struct pic_dict *exports;
 
   if ((lib = pic_find_library(pic, name)) != NULL) {
-
-#if DEBUG
-    printf("* reopen library: ");
-    pic_debug(pic, name);
-    puts("");
-#endif
-
-    return lib;
+    pic_errorf(pic, "library name already in use: ~s", name);
   }
 
   env = pic_null_syntactic_environment(pic);
@@ -295,7 +288,9 @@ pic_lib_define_library(pic_state *pic)
 
   pic_get_args(pic, "o*", &spec, &argc, &argv);
 
-  pic_open_library(pic, spec);
+  if (! pic_find_library(pic, spec)) {
+    pic_make_library(pic, spec);
+  }
 
   pic_try {
     pic_in_library(pic, spec);
diff --git a/extlib/benz/state.c b/extlib/benz/state.c
index c4207987..521b9a0d 100644
--- a/extlib/benz/state.c
+++ b/extlib/benz/state.c
@@ -322,8 +322,8 @@ pic_open(int argc, char *argv[], char **envp, pic_allocf allocf)
   pic->xSTDERR = pic_make_standard_port(pic, xstderr, PIC_PORT_OUT);
 
   /* standard libraries */
-  pic->PICRIN_BASE = pic_open_library(pic, pic_read_cstr(pic, "(picrin base)"));
-  pic->PICRIN_USER = pic_open_library(pic, pic_read_cstr(pic, "(picrin user)"));
+  pic->PICRIN_BASE = pic_make_library(pic, pic_read_cstr(pic, "(picrin base)"));
+  pic->PICRIN_USER = pic_make_library(pic, pic_read_cstr(pic, "(picrin user)"));
   pic->lib = pic->PICRIN_USER;
   pic->prev_lib = NULL;