diff --git a/src/structs.h b/src/structs.h
index 8f554fe..26fee9a 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1249,13 +1249,16 @@
      * The following only used in undo.c.
      */
     u_header_T	*b_u_oldhead;	/* pointer to oldest header */
-    u_header_T	*b_u_newhead;	/* pointer to newest header */
+    u_header_T	*b_u_newhead;	/* pointer to newest header; may not be valid
+				   if b_u_curhead is not NULL */
     u_header_T	*b_u_curhead;	/* pointer to current header */
     int		b_u_numhead;	/* current number of headers */
     int		b_u_synced;	/* entry lists are synced */
     long	b_u_seq_last;	/* last used undo sequence number plus 1 */
     long	b_u_seq_cur;	/* undo sequence number of last header used
 				   plus 1 */
+    time_t	b_u_seq_time;	/* uh_time of the last header used plus 1 or
+				   uh_time of current header */
 
     /*
      * variables for "U" command in undo.c