From c8da9c501c8ad0945499688151b57766af659e15 Mon Sep 17 00:00:00 2001
From: Chris Hanson <org/chris-hanson/cph>
Date: Fri, 5 May 2017 23:18:16 -0700
Subject: [PATCH] Fix bug: incorrect assumption of mutability.

---
 src/runtime/dbgutl.scm | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/runtime/dbgutl.scm b/src/runtime/dbgutl.scm
index 50e6512ab..2edadb2ba 100644
--- a/src/runtime/dbgutl.scm
+++ b/src/runtime/dbgutl.scm
@@ -94,8 +94,9 @@ USA.
 	       (parameterize* (list (cons current-output-port port))
 			      thunk)))))
       (if (and (car x) (> length 4))
-	  (string-copy! (cdr x) (- length 4) " ..."))
-      (cdr x))))
+	  (string-append (string-slice (cdr x) 0 (- length 4))
+			 " ...")
+	  (cdr x)))))
 
 (define (show-frames environment depth port)
   (debugger-presentation port
-- 
2.25.1