Included patch for persistent undo. Lots of changes and added test.
diff --git a/runtime/doc/various.txt b/runtime/doc/various.txt
index cfadfd6..65688ee 100644
--- a/runtime/doc/various.txt
+++ b/runtime/doc/various.txt
@@ -359,6 +359,7 @@
N *+path_extra* Up/downwards search in 'path' and 'tags'
m *+perl* Perl interface |perl|
m *+perl/dyn* Perl interface |perl-dynamic| |/dyn|
+H *+persistent_undo* Persistent undo |undo-persistence|
*+postscript* |:hardcopy| writes a PostScript file
N *+printer* |:hardcopy| command
H *+profile* |:profile| command