diff --git a/runtime/indent/prolog.vim b/runtime/indent/prolog.vim
index ac03c28..0c4fd54 100644
--- a/runtime/indent/prolog.vim
+++ b/runtime/indent/prolog.vim
@@ -4,6 +4,7 @@
 " 		Doug Kearns <dougkearns@gmail.com>
 "  Revised on:	2002.02.18. 23:34:05
 "  Last change by: Takuya Fujiwara, 2018 Sep 23
+"		2022 April: b:undo_indent added by Doug Kearns
 
 " TODO:
 "   checking with respect to syntax highlighting
@@ -21,6 +22,8 @@
 setlocal indentkeys-=:,0#
 setlocal indentkeys+=0%,-,0;,>,0)
 
+let b:undo_indent = "setl inde< indk<"
+
 " Only define the function once.
 "if exists("*GetPrologIndent")
 "    finish