--- a/hgext3rd/topic/revset.py	Tue Mar 10 19:04:33 2020 +0700
+++ b/hgext3rd/topic/revset.py	Tue Mar 10 19:05:08 2020 +0700
@@ -124,6 +124,7 @@
                 b'relation subscript bounds must be integers',
                 None, None)
         else:
+            # hg <= 4.9 (431cf2c8c839+13f7a6a4f0db)
             a = b = z
 
         s = revset.getset(repo, revset.fullreposet(repo), x)