diff --git a/src/ikarus.boot b/src/ikarus.boot
index ea5c7f6..38e4abc 100644
Binary files a/src/ikarus.boot and b/src/ikarus.boot differ
diff --git a/src/ikarus.core.ss b/src/ikarus.core.ss
index a643120..6fdc1ec 100644
--- a/src/ikarus.core.ss
+++ b/src/ikarus.core.ss
@@ -77,35 +77,6 @@
 
 
 
-#|procedure:substring
-  (substring str i j)
-  Returns a substring of str starting from index i (inclusive)
-  and ending with index j (exclusive).|#
-(let ()
-  (define fill
-    (lambda (s d si sj di)
-      (cond
-        [($fx= si sj) d]
-        [else
-         ($string-set! d di ($string-ref s si))
-         (fill s d ($fxadd1 si) sj ($fxadd1 di))])))
-  (primitive-set! 'substring
-     (lambda (s n m)
-       (unless (string? s)
-         (error 'substring "~s is not a string" s))
-       (let ([len ($string-length s)])
-         (unless (and (fixnum? n)
-                      ($fx>= n 0)
-                      ($fx< n len))
-           (error 'substring "~s is not a valid start index for ~s" n s))
-         (unless (and (fixnum? m)
-                      ($fx>= m 0)
-                      ($fx<= m len))
-           (error 'substring "~s is not a valid end index for ~s" m s))
-         (let ([len ($fx- m n)])
-           (if ($fx<= len 0)
-               ""
-               (fill s ($make-string len) n m 0)))))))
 
 (primitive-set! 'not 
   (lambda (x) (if x #f #t)))
diff --git a/src/ikarus.strings.ss b/src/ikarus.strings.ss
index aabbb7f..cb0f042 100644
--- a/src/ikarus.strings.ss
+++ b/src/ikarus.strings.ss
@@ -1,13 +1,13 @@
 
 (library (ikarus strings)
   (export string-length string-ref string-set! make-string string->list string=?
-          string-append)
+          string-append substring)
   (import 
     (except (ikarus) string-length string-ref string-set! make-string
-            string->list string=? string-append)
+            string->list string=? string-append substring)
     (only (scheme) 
           $fx+ $fxsub1 $fxadd1 $char= $car $cdr
-          $fxzero? $fx= $fx<= $fx< $fx>=
+          $fxzero? $fx= $fx<= $fx< $fx>= $fx-
           $string-length $string-ref 
           $make-string $string-set!))
 
@@ -67,6 +67,32 @@
         make-string))
 
 
+  (module (substring)
+    (define fill
+      (lambda (s d si sj di)
+        (cond
+          [($fx= si sj) d]
+          [else
+           ($string-set! d di ($string-ref s si))
+           (fill s d ($fxadd1 si) sj ($fxadd1 di))])))
+    (define substring
+       (lambda (s n m)
+         (unless (string? s)
+           (error 'substring "~s is not a string" s))
+         (let ([len ($string-length s)])
+           (unless (and (fixnum? n)
+                        ($fx>= n 0)
+                        ($fx< n len))
+             (error 'substring "~s is not a valid start index for ~s" n s))
+           (unless (and (fixnum? m)
+                        ($fx>= m 0)
+                        ($fx<= m len))
+             (error 'substring "~s is not a valid end index for ~s" m s))
+           (let ([len ($fx- m n)])
+             (if ($fx<= len 0)
+                 ""
+                 (fill s ($make-string len) n m 0)))))))
+
   (module (string=?)
     (define bstring=?
       (lambda (s1 s2 i j)