undo-equiv change

[Stefan Monnier <monnier@iro.umontreal.ca>]

Could someone explain to me what the recent change in simple.el (appended
below) does?

I'm especially wondering why we turned a check for `equiv' into a check for
(listp equiv), thus allowing nil values through: the following code doesn't
seem to make uch sense when equiv is nil.


        Stefan


2005-04-11  Richard M. Stallman  <rms@gnu.org>

	* simple.el (undo): Record t in undo-equiv-table
	for the redo record made by an undo-in-region.

Index: lisp/simple.el
===================================================================
RCS file: /cvsroot/emacs/emacs/lisp/simple.el,v
retrieving revision 1.708
retrieving revision 1.709
diff -u -r1.708 -r1.709
--- lisp/simple.el	7 Apr 2005 15:15:15 -0000	1.708
+++ lisp/simple.el	11 Apr 2005 18:09:45 -0000	1.709
@@ -1279,7 +1279,9 @@
 (defalias 'advertised-undo 'undo)
 
 (defconst undo-equiv-table (make-hash-table :test 'eq :weakness t)
-  "Table mapping redo records to the corresponding undo one.")
+  "Table mapping redo records to the corresponding undo one.
+A redo record for undo-in-region maps to t.
+A redo record for ordinary undo maps to the following (earlier) undo.")
 
 (defvar undo-in-region nil
   "Non-nil if `pending-undo-list' is not just a tail of `buffer-undo-list'.")
@@ -1339,7 +1341,7 @@
 	  (message (if undo-in-region
 		       (if equiv "Redo in region!" "Undo in region!")
 		     (if equiv "Redo!" "Undo!"))))
-      (when (and equiv undo-no-redo)
+      (when (and (listp equiv) undo-no-redo)
 	;; The equiv entry might point to another redo record if we have done
 	;; undo-redo-undo-redo-... so skip to the very last equiv.
 	(while (let ((next (gethash equiv undo-equiv-table)))
@@ -1350,10 +1352,13 @@
 	 (prefix-numeric-value arg)
        1))
     ;; Record the fact that the just-generated undo records come from an
-    ;; undo operation, so we can skip them later on.
+    ;; undo operation--that is, they are redo records.
+    ;; In the ordinary case (not within a region), map the redo
+    ;; record to the following undos.
     ;; I don't know how to do that in the undo-in-region case.
-    (unless undo-in-region
-      (puthash buffer-undo-list pending-undo-list undo-equiv-table))
+    (puthash buffer-undo-list
+	     (if undo-in-region t pending-undo-list)
+	     undo-equiv-table)
     ;; Don't specify a position in the undo record for the undo command.
     ;; Instead, undoing this should move point to where the change is.
     (let ((tail buffer-undo-list)