--- a/mercurial/localrepo.py	Tue Nov 24 11:20:25 2009 -0800
+++ b/mercurial/localrepo.py	Tue Nov 24 14:32:19 2009 +0200
@@ -128,6 +128,12 @@
             return context.workingctx(self)
         return context.changectx(self, changeid)
 
+    def __contains__(self, changeid):
+        try:
+            return bool(self.lookup(changeid))
+        except error.RepoLookupError:
+            return False
+
     def __nonzero__(self):
         return True