--- a/mercurial/cmdutil.py	Tue Jan 02 16:58:37 2018 +0900
+++ b/mercurial/cmdutil.py	Tue Jan 02 17:00:48 2018 +0900
@@ -2352,7 +2352,7 @@
     '_patsfollow':      ('follow(%(val)r)', ' or '),
     '_patsfollowfirst': ('_followfirst(%(val)r)', ' or '),
     'keyword':          ('keyword(%(val)r)', ' or '),
-    'prune':            ('not (%(val)r or ancestors(%(val)r))', ' and '),
+    'prune':            ('not ancestors(%(val)r)', ' and '),
     'user':             ('user(%(val)r)', ' or '),
 }