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(defun my/eww-rename-buffer-as-title ()
  (interactive)
  (when (eq major-mode 'eww-mode)
    (rename-buffer (format "*eww (%s)*" eww-current-title) t)))
;;; ↓ だとタイミングが早すぎるので、
;;;    やるなら defadvice の after がいいかなと思う
;; (add-hook 'eww-mode-hook 'my/eww-rename-buffer-as-title)