If the operator names a macro, its associated macro function is applied to the entire form and the result of that application is used in place of the original form.
Specifically, a symbol names a macro in a given lexical environment if macro-function is true of the symbol and that environment. The function returned by macro-function is a function of two arguments, called the expansion function. The expansion function is invoked by calling the macroexpand hook with the expansion function as its first argument, the entire macro form as its second argument, and an environment object (corresponding to the current lexical environment) as its third argument. The macroexpand hook, in turn, calls the expansion function with the form as its first argument and the environment as its second argument. The value of the expansion function, which is passed through by the macroexpand hook, is a form. The returned form is evaluated in place of the original form.
The consequences are undefined if a macro function destructively modifies any part of its form argument.
A macro name is not a function designator, and cannot be used as the function argument to functions such as apply, funcall, or map.
An implementation is free to implement a Common Lisp special operator as a macro. An implementation is free to implement any macro operator as a special operator, but only if an equivalent definition of the macro is also provided.
The next figure lists some defined names that are applicable to macros.
*macroexpand-hook* macro-function macroexpand-1 defmacro macroexpand macrolet
Figure 3-3. Defined names applicable to macros