+ "Non-nil means compare-windows highlights the differences.
+The value t removes highlighting immediately after invoking a command
+other than `compare-windows'.
+The value `persistent' leaves all highlighted differences. You can clear
+out all highlighting later with the command `compare-windows-dehighlight'."
+ :type '(choice (const :tag "No highlighting" nil)
+ (const :tag "Persistent highlighting" persistent)
+ (other :tag "Highlight until next command" t))
+ :group 'compare-windows