Practical Lisp systems provide a mechanism that allows users to add new derived expressions and specify their implementation as syntactic transformations without modifying the evaluator. Such a user-defined transformation is called a /macro/. Although it is easy to add an elementary mechanism for defining macros, the resulting language has subtle name-conflict problems. There has been much research on mechanisms for macro definitions that do not cause these difficulties. See, for example, Kohlbecker 1986, Clinger and
Rees 1991, and Hanson 1991.
(Aside: “practical” lisp systems have them; the dialect covered in the book does not. Students can and do draw the obvious conclusion ….)