diff --git a/src/utility.cl b/src/utility.cl
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--- /dev/null
+++ b/src/utility.cl
@@ -0,0 +1,23 @@
+;;;; CLASH --- The Common Lisp Adaptable Simple HTTP server
+;;;; This is copyrighted software.  See documentation for terms.
+;;;; 
+;;;; utility.cl --- Basic substrate utilities
+;;;; 
+;;;; Checkout Tag: $Name$
+;;;; $Id$
+
+(in-package :CLASH)
+
+;;;; %File Description:
+;;;; 
+;;;; 
+;;;; 
+
+(deftype string-index ()
+  "Type of string indices."
+  `(integer 0 (,array-dimension-limit)))
+
+(deftype string-inner-index (length)
+  "Type of inner string indices, i.e. those that occur at least length
+characters inside the string."
+  `(integer 0 (,(- array-dimension-limit (- length 1)))))