Macros often have associated auxiliary identifiers (sometimes called keywords or reserved words, although both terms are problematic in Racket). For example, cond has else; class has public, private, etc; unit has import and export.

The fundamental question is what constitutes a use of an auxiliary identifier, and there are two reasonable answers: symbolic equality and referential equality. By symbolic equality I mean, for example, that any identifier written with exactly the letters else is accepted as an else auxiliary form. By referential equality I mean any identifier that refers to (using the standard notions of binding, environments, etc) the binding identified as the else binding.

Danny Yoo had an interesting question on the plt-scheme mailing list recently. At first it seemed like your standard non-hygienic, “I want this to mean something in here” macro question. I used to group macros into three levels of “hygienicness”: the hygienic ones, the ones that are morally hygienic in that the names they introduce are based on their input, and the totally non-hygienic ones that have a fixed name that they stick into the program. An example of the middle set is define-struct, and an example of the third set is a loop construct that binds the name yield in its body.

The third class used to offend me from a purist’s (semi-purist?) perspective. But it’s a very reasonable thing to want to do. Consider the class macro and the names it uses to do interesting things: super, public, field, init, and so on. It depends on those particular names.