struct mkd_renderer {
		/* document level callbacks */
		void (*prolog)(struct buf *ob, void *opaque);
		void (*epilog)(struct buf *ob, void *opaque);
	
		/* block level callbacks - NULL skips the block */
		void (*blockcode)(struct buf *ob, struct buf *text, void *opaque);
		void (*blockquote)(struct buf *ob, struct buf *text, void *opaque);
		void (*blockhtml)(struct buf *ob, struct buf *text, void *opaque);
		void (*header)(struct buf *ob, struct buf *text,
...
}

class ApiHandler(BnwWebHandler):
    @defer.inlineCallbacks
    def respond_post(self, cmd_name):
        stime = time.time()
        user = yield objs.User.find_one(
            {'login_key': self.get_argument('login', '')})
        if not cmd_name:
            defer.returnValue(json.dumps(dict(
                ok=True,
                commands=api_handlers.handlers.keys())))
        if cmd_name not in api_handlers.handlers:
            defer.returnValue(json.dumps(dict(
                ok=False,
                desc='unknown command')))