(overlay-put compare-windows-overlay1 'priority 1000))
(overlay-put compare-windows-overlay1 'window w1)
(if compare-windows-overlay2
(move-overlay compare-windows-overlay2 beg2 end2 b2)
(setq compare-windows-overlay2 (make-overlay beg2 end2 b2))
(overlay-put compare-windows-overlay1 'priority 1000))
(overlay-put compare-windows-overlay1 'window w1)
(if compare-windows-overlay2
(move-overlay compare-windows-overlay2 beg2 end2 b2)
(setq compare-windows-overlay2 (make-overlay beg2 end2 b2))