--- a/mercurial/repoview.py	Mon Sep 16 01:08:29 2013 -0700
+++ b/mercurial/repoview.py	Tue Dec 24 17:44:23 2013 -0500
@@ -94,6 +94,11 @@
     return frozenset(xrange(firstmutable, len(cl)))
 
 # function to compute filtered set
+#
+# When addding a new filter you MUST update the table at:
+#     mercurial.branchmap.subsettable
+# Otherwise your filter will have to recompute all its branches cache
+# from scratch (very slow).
 filtertable = {'visible': computehidden,
                'served': computeunserved,
                'immutable':  computemutable,