diff --git a/extlib/benz/blob.c b/extlib/benz/blob.c
index 3e5b7723..cd5be767 100644
--- a/extlib/benz/blob.c
+++ b/extlib/benz/blob.c
@@ -2,8 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <string.h>
-
 #include "picrin.h"
 #include "picrin/blob.h"
 #include "picrin/pair.h"
diff --git a/extlib/benz/bool.c b/extlib/benz/bool.c
index 0b196f51..b102f9de 100644
--- a/extlib/benz/bool.c
+++ b/extlib/benz/bool.c
@@ -2,8 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <string.h>
-
 #include "picrin.h"
 #include "picrin/pair.h"
 #include "picrin/vector.h"
diff --git a/extlib/benz/cont.c b/extlib/benz/cont.c
index b1af6eca..2678fb0b 100644
--- a/extlib/benz/cont.c
+++ b/extlib/benz/cont.c
@@ -2,10 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <setjmp.h>
-#include <string.h>
-#include <stdarg.h>
-
 #include "picrin.h"
 #include "picrin/proc.h"
 #include "picrin/cont.h"
diff --git a/extlib/benz/error.c b/extlib/benz/error.c
index 69c12206..1d950359 100644
--- a/extlib/benz/error.c
+++ b/extlib/benz/error.c
@@ -2,10 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <stdlib.h>
-#include <string.h>
-#include <stdarg.h>
-
 #include "picrin.h"
 #include "picrin/pair.h"
 #include "picrin/proc.h"
diff --git a/extlib/benz/gc.c b/extlib/benz/gc.c
index 4de6e564..c09765fd 100644
--- a/extlib/benz/gc.c
+++ b/extlib/benz/gc.c
@@ -20,10 +20,6 @@
 #include "picrin/record.h"
 #include "picrin/read.h"
 
-#if GC_DEBUG
-# include <string.h>
-#endif
-
 union header {
   struct {
     union header *ptr;
diff --git a/extlib/benz/include/picrin.h b/extlib/benz/include/picrin.h
index 98b96d0c..707a5c2a 100644
--- a/extlib/benz/include/picrin.h
+++ b/extlib/benz/include/picrin.h
@@ -30,10 +30,17 @@ extern "C" {
 
 #include <stddef.h>
 #include <stdbool.h>
-#include <setjmp.h>
 #include <stdint.h>
 #include <limits.h>
+#include <stdarg.h>
+
+#include <stdio.h>
+#include <setjmp.h>
 #include <assert.h>
+#include <string.h>
+#include <stdlib.h>
+#include <math.h>
+#include <ctype.h>
 
 #include "picrin/xvect.h"
 #include "picrin/xhash.h"
diff --git a/extlib/benz/include/picrin/util.h b/extlib/benz/include/picrin/util.h
index afa0e037..6f39b759 100644
--- a/extlib/benz/include/picrin/util.h
+++ b/extlib/benz/include/picrin/util.h
@@ -32,7 +32,6 @@ extern "C" {
 #if GCC_VERSION >= 40500 || __clang__
 # define PIC_UNREACHABLE() (__builtin_unreachable())
 #else
-# include <assert.h>
 # define PIC_UNREACHABLE() (assert(false))
 #endif
 
diff --git a/extlib/benz/include/picrin/xfile.h b/extlib/benz/include/picrin/xfile.h
index 4db6f836..4c96a9f8 100644
--- a/extlib/benz/include/picrin/xfile.h
+++ b/extlib/benz/include/picrin/xfile.h
@@ -5,12 +5,6 @@
 extern "C" {
 #endif
 
-#include <stddef.h>
-#include <stdarg.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
-
 typedef struct {
   int ungot;
   int flags;
diff --git a/extlib/benz/include/picrin/xhash.h b/extlib/benz/include/picrin/xhash.h
index e25302ed..1dadc7ff 100644
--- a/extlib/benz/include/picrin/xhash.h
+++ b/extlib/benz/include/picrin/xhash.h
@@ -9,12 +9,6 @@
 extern "C" {
 #endif
 
-#include <stddef.h>
-#include <stdlib.h>
-#include <string.h>
-#include <stdint.h>
-#include <assert.h>
-
 /* simple object to object hash table */
 
 #define XHASH_INIT_SIZE 11
diff --git a/extlib/benz/include/picrin/xrope.h b/extlib/benz/include/picrin/xrope.h
index 89842de0..20199b85 100644
--- a/extlib/benz/include/picrin/xrope.h
+++ b/extlib/benz/include/picrin/xrope.h
@@ -5,11 +5,6 @@
 extern "C" {
 #endif
 
-#include <stddef.h>
-#include <stdlib.h>
-#include <string.h>
-#include <assert.h>
-
 /* public APIs */
 
 typedef struct xrope xrope;
diff --git a/extlib/benz/include/picrin/xvect.h b/extlib/benz/include/picrin/xvect.h
index 3701205e..a04d227a 100644
--- a/extlib/benz/include/picrin/xvect.h
+++ b/extlib/benz/include/picrin/xvect.h
@@ -9,11 +9,6 @@
 extern "C" {
 #endif
 
-#include <stddef.h>
-#include <stdlib.h>
-#include <string.h>
-#include <assert.h>
-
 typedef struct xvect {
   char *data;
   size_t size, mask, head, tail, width;
diff --git a/extlib/benz/number.c b/extlib/benz/number.c
index be819bbd..6e84dadf 100644
--- a/extlib/benz/number.c
+++ b/extlib/benz/number.c
@@ -2,10 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <math.h>
-#include <limits.h>
-#include <stdlib.h>
-
 #include "picrin.h"
 #include "picrin/string.h"
 #include "picrin/cont.h"
diff --git a/extlib/benz/pair.c b/extlib/benz/pair.c
index b662534a..03621ec1 100644
--- a/extlib/benz/pair.c
+++ b/extlib/benz/pair.c
@@ -2,8 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <stdarg.h>
-
 #include "picrin.h"
 #include "picrin/pair.h"
 
diff --git a/extlib/benz/port.c b/extlib/benz/port.c
index ec61d984..5032104b 100644
--- a/extlib/benz/port.c
+++ b/extlib/benz/port.c
@@ -2,10 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <stdlib.h>
-#include <string.h>
-#include <limits.h>
-
 #include "picrin.h"
 #include "picrin/proc.h"
 #include "picrin/port.h"
diff --git a/extlib/benz/read.c b/extlib/benz/read.c
index 46a056f8..aed121a6 100644
--- a/extlib/benz/read.c
+++ b/extlib/benz/read.c
@@ -2,9 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <ctype.h>
-#include <math.h>
-#include <stdlib.h>
 #include "picrin.h"
 #include "picrin/read.h"
 #include "picrin/error.h"
diff --git a/extlib/benz/state.c b/extlib/benz/state.c
index 75e940f3..3f32e1c5 100644
--- a/extlib/benz/state.c
+++ b/extlib/benz/state.c
@@ -2,8 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <stdlib.h>
-
 #include "picrin.h"
 #include "picrin/gc.h"
 #include "picrin/read.h"
diff --git a/extlib/benz/string.c b/extlib/benz/string.c
index 63f301d8..43514b2d 100644
--- a/extlib/benz/string.c
+++ b/extlib/benz/string.c
@@ -2,8 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <string.h>
-
 #include "picrin.h"
 #include "picrin/string.h"
 #include "picrin/pair.h"
diff --git a/extlib/benz/symbol.c b/extlib/benz/symbol.c
index 115582c8..3fd40f68 100644
--- a/extlib/benz/symbol.c
+++ b/extlib/benz/symbol.c
@@ -2,10 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <string.h>
-#include <stdlib.h>
-#include <math.h>
-
 #include "picrin.h"
 #include "picrin/string.h"
 
diff --git a/extlib/benz/vm.c b/extlib/benz/vm.c
index 66ed3871..ea4821e2 100644
--- a/extlib/benz/vm.c
+++ b/extlib/benz/vm.c
@@ -2,11 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <stdlib.h>
-#include <stdarg.h>
-#include <limits.h>
-#include <math.h>
-
 #include "picrin.h"
 #include "picrin/pair.h"
 #include "picrin/string.h"
diff --git a/extlib/benz/write.c b/extlib/benz/write.c
index c3204dc4..e35d6eee 100644
--- a/extlib/benz/write.c
+++ b/extlib/benz/write.c
@@ -2,8 +2,6 @@
  * See Copyright Notice in picrin.h
  */
 
-#include <math.h>
-
 #include "picrin.h"
 #include "picrin/port.h"
 #include "picrin/pair.h"