--- a/mercurial/revlogutils/__init__.py	Wed Nov 06 14:13:19 2019 +0100
+++ b/mercurial/revlogutils/__init__.py	Wed Nov 06 14:13:19 2019 +0100
@@ -0,0 +1,14 @@
+# mercurial.revlogutils -- basic utilities for revlog
+#
+# Copyright 2019 Pierre-Yves David <pierre-yves.david@octobus.net>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+from .. import error
+
+
+class NodeMap(dict):
+    def __missing__(self, x):
+        raise error.RevlogError('unknown node: %s' % x)