Common Lisp the Language, 2nd Edition

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### 28.1.7.3. Declarative Method Combination

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 28.1.7.2. 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.