From 9974c3bf7eb0b6205c99eeaef9f10a2336e500bc Mon Sep 17 00:00:00 2001
From: Matthew Flatt <mflatt@racket-lang.org>
Date: Sat, 2 Feb 2019 14:12:04 -0700
Subject: [PATCH] repair to single-valued tracking

original commit: 8eb7644eb5def8d2a0864ef71610457d8be8c111
---
 s/cp0.ss | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/s/cp0.ss b/s/cp0.ss
index 200a28d970..bd5b48f433 100644
--- a/s/cp0.ss
+++ b/s/cp0.ss
@@ -962,7 +962,9 @@
                  (if (pair? val)
                      (car val)
                      (let ([r (pred?)])
-                       (set-cdr! a (cons r (cdr a)))
+                       (let ([p (cdr a)])
+                         (unless (pair? p)
+                           (set-cdr! a (cons r p))))
                        r)))))]))
 
       (define-syntax with-memoize