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.

>>17
Well, full-scale languages do support mutually recursive definitions, e.g. OCaml via let rec. You're saying it's hard as hell to do and I shouldn't bother for my toy language?