创建一个 help 缓冲，把 [back] 按钮的属性读出来，发现其中 category 的值 help-back-button（是一个符号），然而直接把它跟 help-back-button 比较的时候返回 nil：
(let ((curr-buf (current-buffer))
(help-buf "*Help2*")
(shouldeq (lambda (testfn sym1 sym2)
(format "(testfn %S %S) => %s\n" sym1 sym2 (funcall testfn sym1 sym2)))))
(with-current-buffer (get-buffer-create help-buf)
(read-only-mode -1)
(erase-buffer)
(help-insert-xref-button help-back-label 'help-back curr-buf)
(help-mode)
(backward-char)
(concat (format "properties: %S\n" (text-properties-at (point)))
(funcall shouldeq 'equal (get-text-property (point) 'help-args) (list curr-buf))
(funcall shouldeq 'eq (get-text-property (point) 'type) 'help-back)
(funcall shouldeq 'eq (get-text-property (point) 'category) 'help-back-button)
(funcall shouldeq 'equal (symbolp (get-text-property (point) 'category)) t)
)))
;; =>
;; "properties: (help-args (#<buffer *scratch*>) category help-back-button button (t) fontified nil)
;; (testfn (#<buffer *scratch*>) (#<buffer *scratch*>)) => t
;; (testfn help-back help-back) => t
;; (testfn help-back-button help-back-button) => nil
;; (testfn t t) => t
;; "
另一个比较疑惑的地方是：(text-properties-at ...) 取出来的 properties 并不包含 type，而它其实是存在的，(help-insert-xref-button ...) 第二个参数就是 type，通过 (get-text-property ...) 也可以取到，为何要这样设计？
APPEND 2018-09-16 00.53.09
(button-category-symbol 'help-back) ;; => help-back-button
(symbolp (button-category-symbol 'help-back)) ;; => t
(eq (button-category-symbol 'help-back) 'help-back-button) ;; => nil
(equal (button-category-symbol 'help-back) 'help-back-button) ;; => nil