(xcb:+request+reply connection
                   (make-instance 'xcb:ewmh:get-_NET_ACTIVE_WINDOW
                                  :window root)))

(slot-value connection 'reply-plist)
→ (96 (xcb:ewmh:get-_NET_ACTIVE_WINDOW [1 32 96 0 2 0 0 0 33 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 154 1 64 5 0 0 0 0]))

(setq reply-plist (slot-value connection 'reply-plist))
→ (96 (xcb:ewmh:get-_NET_ACTIVE_WINDOW [1 32 96 0 2 0 0 0 33 0 0 0 ...]))

(setq reply-data (plist-get reply-plist 96))
→ (xcb:ewmh:get-_NET_ACTIVE_WINDOW [1 32 96 0 2 0 0 0 33 0 0 0 ...])

(setq class-name (xcb:-request-class->reply-class (car reply-data)))
→ xcb:ewmh:get-_NET_ACTIVE_WINDOW~reply

(setq reply-data (cadr reply-data)
      replies (make-instance class-name))

(xcb:unmarshal replies reply-data)
→ error

(cl-assert (= 1 (length (slot-value replies 'value))))
→ error

(length (slot-value replies 'value))
→ 2
(slot-value replies 'type)
→ 33
(slot-value replies 'format)
→ 32
(slot-value replies 'bytes-after)
→ 0
(slot-value replies 'value-len)
→ 2
(slot-value replies 'value)
→ [88080794 0]