Hi,
    I try to understand what the filter system is for and why the
transcoders itself doesn't suffice. So I take an example and want to see
how the filters are used in real exporter.  I look at ox-latex.el and try
to make sense of the filters

1. There are only two filters are used:

 :filters-alist '((:filter-options . org-latex-math-block-options-filter)
   (:filter-parse-tree . org-latex-math-block-tree-filter))

The purpose of org-latex-math-block-options-filter seems to be converting
whatever in the property list of :author, :date, :title  to strings of
latex commands(please correct me if I am wrong).  But what is the purpose
of org-latex-math-block-tree-filter? The org-latex--wrap-latex-math-block
is quite sophisticate and I don't get what it's doing.

-----------------------------------------------------
(defun org-latex-math-block-options-filter (info backend)
  (dolist (prop '(:author :date :title) info)
    (plist-put info prop
       (org-latex--wrap-latex-math-block (plist-get info prop) info))))
----------------------------------------------------
(defun org-latex-math-block-tree-filter (tree backend info)
  (org-latex--wrap-latex-math-block tree info))
----------------------------------------------------

2. For the filters of the form :filter-TYPE, there is also a transcoder for
processing the same TYPE. For example, for bold, there are :filter-bold,
but bold is also processed in the transcoder specified in the transcoder
alist when the backend is defined(by org-export-define-backend). When shall
we use a transcode and when shall we use a filter?

Thank you.

Shiyuan