1 files changed, 11 insertions, 6 deletions
diff --git a/slsets.scm b/slsets.scm
index 92d7356..4b83cf4 100644
--- a/slsets.scm
+++ b/slsets.scm
@@ -78,12 +78,17 @@
 (define (slset-adjoin lst . els)
   (with-marked-list lst
                     (lambda (m)
-                      (let lp ((lst lst)
-                               (els els))
-                        (cond ((null? els) lst)
-                              ((marked? (car els) m) (lp lst (cdr els)))
-                              (else (mark! (car els) m)
-                                    (lp (cons (car els) lst) (cdr els))))))))
+                      (dynamic-wind
+                          void
+                          (lambda ()
+                            (let lp ((lst lst)
+                                     (els els))
+                              (cond ((null? els) lst)
+                                    ((marked? (car els) m) (lp lst (cdr els)))
+                                    (else (mark! (car els) m)
+                                          (lp (cons (car els) lst) (cdr els))))))
+                          (lambda ()
+                            (unmark-list! els m))))))
 
 (define (slset-difference lst . lsts)
   (let lp ((lsts lsts)