Common Lisp the Language, 2nd Edition
The macro define-method-combination defines new forms of method combination. It provides a mechanism for customizing the production of the effective method. The default procedure for producing an effective method is described in section 18.104.22.168. There are two forms of define-method-combination. The short form is a simple facility; the long form is more powerful and more verbose. The long form resembles defmacro in that the body is an expression that computes a Lisp form; it provides mechanisms for implementing arbitrary control structures within method combination and for arbitrary processing of method qualifiers. The syntax and use of both forms of define-method-combination are explained in section 28.2.