"The code that implements Arc illustrates many useful programming techniques. Some of the techniques occur multiple times, and can be considered idioms of Arc programming; techniques that can be applied to many problems. This article describes some of the most common idioms, and how they can be used in Arc programming. The techniques described focus largely on macros and recursion..." - Arc is free software: http://lists.gnu.org/archive/html/emacs-devel/2008-03/msg01117.html