Implement a Lisp macro system

/prog/, do you know of a tutorial or a paper on implementing a Lisp macro system from scratch? It could be Scheme macros, it could be really simple macros, anything would be helpful.

>>12
But you'll have to report a compile error for

(defmacro foo () (bar))
(defmacro bar () (foo))

anyways. You may as well force the programmer to order them.