diff --git a/mercurial/ancestor.py b/mercurial/ancestor.py
--- a/mercurial/ancestor.py
+++ b/mercurial/ancestor.py
@@ -11,6 +11,7 @@ import heapq
 
 from .node import nullrev
 from . import (
+    dagop,
     policy,
     pycompat,
 )
@@ -162,6 +163,9 @@ class incrementalmissingancestors(object
         '''grow the ancestor set by adding new bases'''
         self.bases.update(newbases)
 
+    def basesheads(self):
+        return dagop.headrevs(self.bases, self.pfunc)
+
     def removeancestorsfrom(self, revs):
         '''remove all ancestors of bases from the set revs (in place)'''
         bases = self.bases