+If <var>service</var> is specified, it returns available list of
+Content-Transfer-Encoding for it.
+</defun>
+
+
+<h2> How to write encoder/decoder module
+<node> mel-backend
+<p>
+<defmacro name="mel-define-method">
+<args> name args <rest> body
+<p>
+Define <var>name</var> as a method function of (nth 1 (car (last
+<var>args</var>))) backend.
+<p>
+<var>args</var> is like an argument list of lambda, but (car (last
+<var>args</var>)) must be specialized parameter. (car (car (last
+<var>args</var>))) is name of variable and (nth 1 (car (last
+<var>args</var>))) is name of backend (encoding).
+<p>
+Example:
+<p>
+<lisp>
+(mel-define-method mime-write-decoded-region (start end filename
+ (nil "base64"))
+ "Decode and write current region encoded by base64 into FILENAME.
+START and END are buffer positions."
+ (interactive
+ (list (region-beginning) (region-end)
+ (read-file-name "Write decoded region to file: ")))
+ (let ((str (buffer-substring start end)))
+ (with-temp-buffer
+ (insert (decode-base64-string str))
+ (write-region-as-binary (point-min) (point-max) filename)
+ )))
+</lisp>
+</defmacro>
+
+<defmacro name="mel-define-method-function">
+<args> spec function
+<p>
+Set <var>spec</var>'s function definition to <var>function</var>.
+<p>
+First element of <var>spec</var> is service.
+<p>
+Rest of <var>args</var> is like an argument list of lambda, but (car
+(last <var>args</var>)) must be specialized parameter. (car (car
+(last <var>args</var>))) is name of variable and (nth 1 (car (last
+<var>args</var>))) is name of backend (encoding).
+<p>
+Example:
+<p>
+<lisp>
+(mel-define-method-function (mime-encode-string string (nil "base64"))
+ 'encode-base64-string)
+</lisp>
+</defmacro>
+
+
+<h2> How to add encoding/decoding service
+<node> generic function for mel-backend
+<p>
+<defmacro name="mel-define-service">
+<args> name
+<opts> args doc-string
+<p>
+Define <var>name</var> as a service for Content-Transfer-Encodings.
+<p>
+If <var>args</var> is specified, <var>name</var> is defined as a
+generic function for the service.
+<p>
+Example:
+<p>
+<lisp>
+(mel-define-service encoded-text-encode-string (string encoding)
+ "Encode STRING as encoded-text using ENCODING.
+ENCODING must be string.")
+</lisp>
+</defmacro>