From 1e76e39b9a9cf50bdfb3545541b8ecfa03d87a6e Mon Sep 17 00:00:00 2001
From: Alex Shinn <alexshinn@gmail.com>
Date: Mon, 16 May 2016 22:56:51 +0900
Subject: [PATCH] don't allow redefining reader labels

Fixes issue #337.
---
 lib/srfi/38.scm | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/lib/srfi/38.scm b/lib/srfi/38.scm
index 9e8069b7..b8e79fa7 100644
--- a/lib/srfi/38.scm
+++ b/lib/srfi/38.scm
@@ -320,7 +320,10 @@
             ((#\0 #\1 #\2 #\3 #\4 #\5 #\6 #\7 #\8 #\9)
              (let* ((str (read-label '()))
                     (n (string->number str)))
-               (if (not n) (read-error "read error: invalid reference" str))
+               (if (not n)
+                   (read-error "read error: invalid reference" str))
+               (if (assv n shared)
+                   (read-error "read error: duplicate label" str))
                (cond
                 ((eqv? #\= (peek-char in))
                  (read-char in)