Index: contrib/bazaar/bzreml/__init__.py
===================================================================
--- contrib/bazaar/bzreml/__init__.py	(revision b553acc54cb92b44efceab64a5bfd014959d54d2)
+++ contrib/bazaar/bzreml/__init__.py	(revision 7cd9b022268fa4ce2acf8f2e5dbb5e5eae9c0bd5)
@@ -102,8 +102,15 @@
 	return ""
 
+def iter_reverse_revision_history(repository, revision_id):
+	"""Iterate backwards through revision ids in the lefthand history"""
+	
+	graph = repository.get_graph()
+	stop_revisions = (None, _mod_revision.NULL_REVISION)
+	return graph.iter_lefthand_ancestry(revision_id, stop_revisions)
+
 def revision_sequence(branch, revision_old_id, revision_new_id):
 	"""Calculate a sequence of revisions"""
 	
-	for revision_ac_id in branch.repository.iter_reverse_revision_history(revision_new_id):
+	for revision_ac_id in iter_reverse_revision_history(branch.repository, revision_new_id):
 		if (revision_ac_id == revision_old_id):
 			break
Index: contrib/bazaar/mbprotect/__init__.py
===================================================================
--- contrib/bazaar/mbprotect/__init__.py	(revision b553acc54cb92b44efceab64a5bfd014959d54d2)
+++ contrib/bazaar/mbprotect/__init__.py	(revision 7cd9b022268fa4ce2acf8f2e5dbb5e5eae9c0bd5)
@@ -45,4 +45,11 @@
 from bzrlib.errors import TipChangeRejected
 
+def iter_reverse_revision_history(repository, revision_id):
+	"""Iterate backwards through revision ids in the lefthand history"""
+	
+	graph = repository.get_graph()
+	stop_revisions = (None, _mod_revision.NULL_REVISION)
+	return graph.iter_lefthand_ancestry(revision_id, stop_revisions)
+
 def pre_change_branch_tip(params):
 	repo = params.branch.repository
@@ -54,5 +61,5 @@
 	# First permitted case is appending changesets to main branch.Look for
 	# old tip in new main branch.
-	for revision_id in repo.iter_reverse_revision_history(params.new_revid):
+	for revision_id in iter_reverse_revision_history(repo, params.new_revid):
 		if revision_id == params.old_revid:
 			return	# Found old tip
@@ -60,5 +67,5 @@
 	# Another permitted case is backing out changesets. Look for new tip
 	# in old branch.
-	for revision_id in repo.iter_reverse_revision_history(params.old_revid):
+	for revision_id in iter_reverse_revision_history(repo, params.old_revid):
 		if revision_id == params.new_revid:
 			return	# Found new tip