/* Extract the fields of a position. */
#define POSN_WINDOW(posn) (XCAR (posn))
#define POSN_BUFFER_POSN(posn) (XCAR (XCDR (posn)))
+#define POSN_BUFFER_SET_POSN(posn,x) (XSETCAR (XCDR (posn), (x)))
#define POSN_WINDOW_POSN(posn) (XCAR (XCDR (XCDR (posn))))
#define POSN_TIMESTAMP(posn) \
(XCAR (XCDR (XCDR (XCDR (posn)))))