--- a/tests/test-topic-stack.t	Sat Dec 22 01:29:59 2018 -0500
+++ b/tests/test-topic-stack.t	Sun Dec 16 11:22:04 2018 +0800
@@ -373,6 +373,17 @@
   hg: parse error: stack takes no arguments, it works on current topic
   [255]
 
+Stack relation subscript:
+
+  $ hg log -r 'foo#stack[0]'
+  1 default {} public c_b
+  $ hg log -r 's0 and foo#stack[0]'
+  1 default {} public c_b
+  $ hg log -r 'foo#stack[4]'
+  5 default {foo} draft c_f
+  $ hg log -r 's4 and foo#stack[4]'
+  5 default {foo} draft c_f
+
 Case with multiple heads on the topic
 -------------------------------------