diff --git a/mx_util.c b/mx_util.c
index 8b67515..0aea318 100644
--- a/mx_util.c
+++ b/mx_util.c
@@ -1268,6 +1268,7 @@ int mx_daemon(int nochdir, int noclose)
     return daemon(nochdir, noclose);
 }
 
+/* guarantee stable sort */
 void _mx_sort_linked_list (void **list,  int (*cmp)(void *o1,void *o2), void ** getnextptr(void *o)) {
 
     void *unsorted=*list;
diff --git a/test_mx_util.c b/test_mx_util.c
index 6f47309..20b58c8 100644
--- a/test_mx_util.c
+++ b/test_mx_util.c
@@ -489,6 +489,7 @@ static void test_listsort(void)
     }
 
     /* (100 0 1 2 50 50 2 1 0 100)  -> ( 0 0 1 1 2 2 50 50 100 100) stable */
+    /* also verify stable sort */
     for (int i=0;i<10;i++) {
         o[i].next= i==9 ? NULL : &o[i+1];
     }